[PGO]Implement metadata combine for 'branch_weights' of direct

callsites when none of the instructions folds the rest away.

- Merge cases are added for simplify-cfg {sink,hoist}, based on https://gcc.godbolt.org/z/avGvc38W7 and https://gcc.godbolt.org/z/dbWbjGhaE
- When one instruction folds the others in, do not update branch_weights
  with sum (see test/Transforms/GVN/calls-readonly.ll)

Differential Revision: https://reviews.llvm.org/D148877