Avoid frame identity for leaf Wasm funclets

[davidwrighton]

Wasm funclet frames currently store their function-table index even when the funclet cannot call another function. The index identifies a suspended caller during stack walking, so call-free funclets do not need it.

This change determines whether each funclet can emit a call from lowered IR before code generation. Call-free funclets retain their frame and virtual-IP tracking for EH, but omit the function-identity store. Calling funclets and root functions are unchanged. Call emission also retains a Checked assertion and a Release backstop so future implicit-call paths cannot leave the identity slot uninitialized.

Measured on a Release browser Wasm System.Private.CoreLib build:

Measurement	Before	After	Difference
System.Private.CoreLib.wasm	28,499,159 bytes	28,498,071 bytes	-1,088 bytes
Wasm Code section	14,148,244 bytes	14,147,115 bytes	-1,129 bytes

Disassembly classification found all 162 call-free funclets omitted the identity store, while all 1,833 calling funclets retained it.

Validation:

• ./build.sh -os browser -c Release -subset clr+libs
• ./build.sh clr.wasmjit+clr.corelib+clr.nativecorelib -os browser -c Checked
• JIT formatting and git diff --check

Resolves #134597

Note

This pull request description was generated with GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 6 pipeline(s).
10 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.