[LICM] Refactor to expose functionality as utility functions

This refactors the core functionality of LICM: HoistRegion, SinkRegion and
PromoteAliasSet (renamed to promoteLoopAccessesToScalars) as utility functions
in LoopUtils. This will enable other transformations to make use of them
directly.

Patch by Ashutosh Nema.

llvm-svn: 230178