Cleanup some of the intrinsic fallback implementations to make it easier to add support for new types

[tannergooding]

This adds a couple helpers to the internal Scalar<T> type making it easier to centralize type classification and simplifies various implementations to use those, making it easier to add new types (char and Half) in the future.

It also simplifies the Shuffle/ShuffleNative implementation to go through a single centralized internal method, ensuring we don't have a bunch of complex logic unnecessarily duplicated and that all types are properly supported. This last part means that it adds the missing nint/nuint shuffle overloads which were approved as part of the general expansion to support those types. The two proposals (shuffle and nint/nuint) went up at similar times and we had done a blanket approval for the latter (across 3 proposals), so it was just missed.

The ref assembly sources had also been manually updated on a number of occasions, so this regenerates them (which also alphabetized some of the entries).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime-intrinsics
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR refactors several SIMD intrinsic fallback implementations to centralize scalar type classification (via Scalar<T>) and to de-duplicate Shuffle / ShuffleNative fallback logic across the Vector64/128/256/512 and Vector<T> APIs. It also regenerates the System.Numerics.Vectors ref source.

Changes:

• Introduce Scalar<T>.IsSupported, IsUnsigned, and IsFloatingPoint helpers and update multiple intrinsic fallbacks to use them.
• Consolidate Shuffle / ShuffleNative fallback implementations via shared generic helpers (per vector width) to reduce duplicated per-type code.
• Regenerate System.Numerics.Vectors ref sources (mostly formatting/alphabetization).

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Scalar.cs	Adds centralized scalar type classification helpers and simplifies some scalar ops to reuse existing helpers.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector64.cs	Uses Scalar<T> helpers for classification and centralizes Shuffle / ShuffleNative fallback logic.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector64_1.cs	Switches IsSupported to Scalar<T>.IsSupported.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128.cs	Uses Scalar<T> helpers and consolidates Shuffle / ShuffleNative logic through shared helpers.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector128_1.cs	Switches IsSupported to Scalar<T>.IsSupported.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector256.cs	Uses Scalar<T> helpers and centralizes Shuffle / ShuffleNative fallback logic.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector256_1.cs	Switches IsSupported to Scalar<T>.IsSupported.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector512.cs	Uses Scalar<T> helpers and centralizes Shuffle / ShuffleNative fallback logic.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Vector512_1.cs	Switches IsSupported to Scalar<T>.IsSupported.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/ISimdVector_2.cs	Removes an unused CodeAnalysis using.
src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/SimdVectorExtensions.cs	Removes unused usings.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector.cs	Replaces repeated typeof(T) classification checks with Scalar<T> helpers in multiple ops.
src/libraries/System.Private.CoreLib/src/System/Numerics/Vector_1.cs	Switches IsSupported to Scalar<T>.IsSupported.
src/libraries/System.Numerics.Vectors/ref/System.Numerics.Vectors.cs	Regenerated ref source (primarily type name formatting / ordering).

[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 6 comments.

[tannergooding]

Sometimes copilot suggestions are a space off and it just tries to correct itself after the fact -.-

Ended up fixing the indentation manually to be correct.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 15 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 11 changed files in this pull request and generated 1 comment.

[tannergooding]

Figured out the mono interpreter failure. Will do a follow up PR to resolve the problem so that IsSupported can still be centralized, have undone that part of the change for getting the bigger simplification in.