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Peter Rong
1aebdd72d2
SwiftMergeFunctions replaces merged function bodies with tail-call
trampolines that have different parameter counts than the original.
This breaks the musttail contract which requires the caller and callee
to have exactly matching parameter counts and types.
The fix excludes any function containing a musttail call from merge
candidates. Without this, structurally similar functions with musttail
calls (e.g. ObjC direct method precondition thunks) get merged into
trampolines that produce invalid LLVM IR ("input module is broken").