[PGO] Make indirect call promotion a utility
This patch factors out the main code transformation utilities in the pgo-driven
indirect call promotion pass and places them in Transforms/Utils. The change is
intended to be a non-functional change, letting non-pgo-driven passes share a
common implementation with the existing pgo-driven pass.
The common utilities are used to conditionally promote indirect call sites to
direct call sites. They perform the underlying transformation, and do not
consider profile information. The pgo-specific details (e.g., the computation
of branch weight metadata) have been left in the indirect call promotion pass.
Differential Revision: https://reviews.llvm.org/D40658