[Wasm Ryujit] Flow cannot fall through into a throw helper

[AndyAyersMS]

Make sure we emit proper Wasm block/end for throw helpers, since we may branch to them mid-block, so can't fall through even if the helper is the next block.

Also make it clearer which blocks are throw helpers, instead of trying to guess by parsing block contents and such.

[AndyAyersMS]

@adamperlin PTAL
fyi @dotnet/jit-contrib

Some minor diffs in non-Wasm targets, mainly throw helper blocks getting reordered.

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[Copilot]

Pull request overview

This PR fixes a Wasm RyuJIT issue where control flow could incorrectly fall through into throw helper blocks. The solution introduces a new BBF_THROW_HELPER flag to explicitly mark throw helper blocks, replacing complex heuristic-based detection logic with a simple flag check.

Changes:

• Introduces BBF_THROW_HELPER flag to explicitly mark throw helper blocks
• Simplifies fgIsThrowHlpBlk() from ~60 lines of complex logic to a single flag check
• Updates Wasm control flow to prevent fall-through to throw helper blocks

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/block.h	Adds new BBF_THROW_HELPER flag and includes it in BBF_SPLIT_NONEXIST to prevent splitting throw helper blocks
src/coreclr/jit/flowgraph.cpp	Sets BBF_THROW_HELPER flag when creating throw helper blocks and adds assertions to verify flag consistency
src/coreclr/jit/compiler.hpp	Replaces complex ~60-line heuristic-based throw helper detection with simple flag check
src/coreclr/jit/fgwasm.cpp	Updates control flow logic to prevent fall-through to throw helper blocks by checking the flag

[adamperlin]

Looks good to me, just one small comment!

[AndyAyersMS]

/ba-g infrastructure abandoned some work