JIT: fix wasm conditional fallthrough

[AndyAyersMS]

An adjacent conditional target is not always a valid wasm fallthrough: closing a Try or ExnRefWrapper interval can inject instructions before the target. Reuse the existing wasm fallthrough check for conditional branches.

Fixes #133465
Fixes #133469

Note

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🟢 Approval recommended

The change correctly reuses the existing wasm fallthrough safety logic to prevent invalid fallthroughs at Try/ExnRefWrapper boundaries, with no apparent behavioral regressions outside wasm.

Pull request overview

This PR fixes WebAssembly codegen for conditional branches by ensuring “adjacent target” is only treated as a valid fallthrough when it’s actually safe in the presence of wasm EH interval boundary emissions.

Changes:

• Update wasm genCodeForJTrue to use BasicBlock::CanRemoveJumpToTarget instead of a raw falseTarget != block->Next() adjacency check.
• Extend BasicBlock::CanRemoveJumpToTarget to cover both BBJ_ALWAYS and BBJ_COND, and incorporate the existing wasm interval end check to block unsafe fallthroughs (Try / ExnRefWrapper ends).

File summaries

File	Description
src/coreclr/jit/codegenwasm.cpp	Use CanRemoveJumpToTarget when deciding whether to emit the unconditional branch to the false target.
src/coreclr/jit/block.cpp	Generalize CanRemoveJumpToTarget and reuse it from CanRemoveJumpToNext, including the wasm interval-end fallthrough prohibition.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

[AndyAyersMS]

fyi @dotnet/wasm-contrib
@jakobbotsch PTAL

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[Copilot]

🔵 Needs a closer look

It changes CoreCLR JIT control-flow emission semantics for wasm (high correctness impact) and should be validated by a maintainer with wasm R2R test coverage signals.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0 new
• Review effort level: Lite