JIT: fix three wasm R2R codegen bugs

[AndyAyersMS]

Found by ReadyToRun-compiling and running the src/tests/JIT test tree on wasm.

• fgUpdateFlowGraph: don't reverse a wasm try/catch header's GT_WASM_JEXCEPT. Its edge polarity is load-bearing for try_table codegen (false = try entry, true = catch-resumption dispatcher).
• valuenum PeelOffsets: read ref/byref constant offsets as host size_t. They are stored as size_t; reading them as target_ssize_t is a wrong-sized read on wasm.
• Vector Get/WithElement import: fall back to the throwing software implementation for an out-of-range constant lane index on wasm; a non-constant index still uses the jump-table lowering.

[AndyAyersMS]

@adamperlin PTAL
@EgorBo for the VN change

fyi @dotnet/wasm-contrib

These are crossgen2 failures -- there are also 3 runtime failures I'm still investigating.

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[Copilot]

Pull request overview

This PR adjusts CoreCLR JIT behavior to fix several WebAssembly ReadyToRun (R2R) codegen issues encountered when compiling/running the JIT test suite on wasm.

Changes:

• Fixes ValueNumStore::PeelOffsets constant offset peeling to correctly read ref/byref VN constants using host-sized storage on wasm.
• Adds wasm-specific importer fallback for Vector.GetElement / Vector.WithElement when a constant lane index is out of range (so software path can throw).
• Prevents fgUpdateFlowGraph from reversing wasm try/catch header GT_WASM_JEXCEPT, preserving edge polarity needed for try_table codegen.

Reviewed changes

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

File	Description
src/coreclr/jit/valuenum.cpp	Updates VN offset peeling to handle ref/byref constant storage sizing correctly on wasm.
src/coreclr/jit/hwintrinsic.cpp	Adds wasm-only handling to avoid generating invalid extract/replace lane encodings for out-of-range constant indices.
src/coreclr/jit/fgopt.cpp	Disables a jump-reversal optimization for wasm try/catch headers to preserve GT_WASM_JEXCEPT semantics for codegen.

Suppressed comments (1)

src/coreclr/jit/valuenum.cpp:16414

• Same issue as the previous branch: BYREF constants still use ConstantValue<target_ssize_t> (wrong-sized read on wasm), and REF coercion should use size_t (the only supported coercion for TYP_REF).

            *offset += varTypeIsGC(TypeOfVN(app.GetArg(1)))
                           ? (target_ssize_t)CoercedConstantValue<ssize_t>(app.GetArg(1))
                           : ConstantValue<target_ssize_t>(app.GetArg(1));

[adamperlin]

varTypeIsGC should return true for TYP_BYREF as well, right? I'm not sure about this comment.