Unify O2K_CHECKED_BOUND_ADD_CNS and O2K_VN

[EgorBo]

I decided that it makes no sense to have these separate. So now the semantics of O2K_VN_ADD_CNS:

1. Decomposed "X+CNS" into op2.GetVN() being X and CNS being op2.GetCNS(). Exactly like O2K_CHECKED_BOUND_ADD_CNS used to be. The decomposition allows us to avoid calling GetVNFunc in the hot loop in MergeEdgeAssertions since we usually are more interested in that X rather than the whole ADD's VN.
2. O2K_VN_ADD_CNS alone doesn't imply that anything is non-negative and no callers assume that (we used to have a bug where IsVNCheckedBound was assumed to be unconditionally never-negative). There is GetOp2().IsVNNeverNegative() which only exists because bound check node actually produce two assertions we can't spawn today because trees may only have 1 assertion. I'm thinking of solving this by introducing an aggregate (or just allow trees to have 2 assertions), or just add a fake COMMA
3. I intentionally tried to make this almost 0 diffs, but there are just a few improvements I couldn't shut (they're not related to the non-negativitiy).

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[Copilot]

Pull request overview

This PR refactors the JIT's assertion descriptor types by collapsing the two related op2 kinds — O2K_CHECKED_BOUND_ADD_CNS and O2K_VN — into a single, unified O2K_VN_ADD_CNS. The new kind always decomposes op2 as vn + cns, with an explicit IsVNNeverNegative() flag carried separately. This removes a footgun where "checked bound" was previously (incorrectly) treated by some callers as implying non-negativity, and simplifies the assertion-table bookkeeping for VN-vs-VN relop assertions in global propagation.

Changes:

• Replace O2K_CHECKED_BOUND_ADD_CNS and O2K_VN with a single O2K_VN_ADD_CNS; rename GetCheckedBound/GetCheckedBoundConstant/IsCheckedBoundNeverNegative to GetVN/GetCns/IsVNNeverNegative and update CreateForArrLen/CreateCompareCheckedBound/CreateRelopVN and Equals accordingly.
• Update RangeCheck::MergeEdgeAssertionsWorker to use the new accessors and add an explicit IsVNCheckedBound(GetVN()) guard on the branch that derives a keBinOpArray limit (since arbitrary VNs from CreateRelopVN now also flow through O2K_VN_ADD_CNS).
• In optAssertionPropGlobal_RelOp, restrict the direct relop-match to cns == 0 for O2K_VN_ADD_CNS and consolidate the duplicate-registration logic in optAddAssertion to a single branch.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/compiler.h	Unifies the two op2 kinds into O2K_VN_ADD_CNS, renames accessors/fields, updates IsBoundsCheckNoThrow, Equals, and the three factory helpers.
src/coreclr/jit/assertionprop.cpp	Updates print/debug/complement/dedup paths to the new kind; adds cns == 0 guard for the direct relop-match in global prop.
src/coreclr/jit/rangecheck.cpp	Switches all MergeEdgeAssertionsWorker branches to the new accessors and adds IsVNCheckedBound guard on the keBinOpArray branch to avoid feeding arbitrary VNs into length-shaped Limits.

[EgorBo]

PTAL @jakobbotsch cc @dotnet/jit-contrib just a clean up, minimal diffs.

[EgorBo]

/ba-g timeout