[llvm] [MergeFunc] Preserve observable function pointer identity (PR #213604)

[Gauarv Chaudhary via llvm-commits]

ANAMASGARD wrote:

@nikic Thank you so much for the feedback I have  addressed review :
- removed stripPointerCasts
- second isCallee is an assert
- tests regenerated with update_test_checks.py
- dropped ValueComparisonKind; use cmpValuesAllowingSelfRef instead

https://github.com/llvm/llvm-project/pull/213604