JIT: restore type-based upper bound for RSZ in range analysis

[EgorBo]

fix a regression in range check elimination found by @saucecontrol for:

public static int M(int i)
{
    ReadOnlySpan<int> vals = [0, 1];
    return vals[i >>> 31]; // RNGCHK was no longer eliminated
}

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[Copilot]

Pull request overview

This PR adjusts CoreCLR JIT range analysis for logical right shifts (GT_RSZ) to recover tighter, type-based upper bounds so bounds-check elimination opportunities aren’t lost when the shift count is known but the shifted value’s range isn’t a helpful constant.

Changes:

• Extends RangeOps::ShiftRight to accept an op1BitWidth and uses it (for logical shifts) to compute a type-based upper bound UMAX(op1Type) >> minShift and to force a non-negative lower bound when shifting by at least 1.
• Passes the shift operand’s bit width from RangeCheck::ComputeRangeForBinOp when analyzing GT_RSZ.
• Adds JIT tests asserting bounds checks are elided for i >>> 31 indexing into a 2-element span and i >>> 28 indexing into a 16-element span.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tests/JIT/opt/RangeChecks/ElidedBoundsChecks.cs	Adds regression coverage for bounds-check elimination on logical right shift–derived indices.
src/coreclr/jit/rangecheck.h	Enhances RangeOps::ShiftRight with an optional operand bit-width parameter and type-based bound derivation for logical shifts.
src/coreclr/jit/rangecheck.cpp	Supplies operand bit width to ShiftRight for GT_RSZ range computation.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.