[llvm][ir] Purge MD_prof custom accessors

This commit purges direct accesses to MD_prof metadata and replaces them
with the accessors provided from the utility file wherever possible.
This commit can be seen as the first step towards switching the branch weights to 64 bits.
See post here: https://discourse.llvm.org/t/extend-md-prof-branch-weights-metadata-from-32-to-64-bits/67492

Reviewed By: davidxl, paulkirth

Differential Revision: https://reviews.llvm.org/D141393