[MergeFunctions] Preserve entry counts on folds (#202218)
**Summary**
`MergeFunctions` can fold equivalent functions into a single retained
implementation. When that happens, the retained body may be reached by
callers of both original functions, but its `function_entry_count`
metadata previously preserved only one side of the profile data.
For example, folding functions with entry counts `2000` and `1000` could
leave the retained body with only `2000`. This patch updates the
retained implementation after a successful merge, so the entry count
becomes `3000`, using saturating add.
For ODR/double-thunk merges, the private backing body gets the combined
count while the thunks keep their own entry counts. For alias-backed
merges, the backing function carries the combined count.
**AI Assistance Disclosure**
AI tools were used to browse the LLVM codebase and explore test-case
variations. All outputs were manually reviewed and validated, and I take
full responsibility for the final changes.
