[CGData] Refactor Global Merge Functions (#115750)
This is a follow-up PR to refactor the initial global merge function
pass implemented in #112671.
It first collects stable functions relevant to the current module and
iterates over those only, instead of iterating through all stable
functions in the stable function map.
This is a patch for
https://discourse.llvm.org/t/rfc-global-function-merging/82608.
