[InstCombine] add helper function for folds of icmp (shl 1, Y), C; NFCI

Clean up the existing code by:
1. Renaming variables
2. Adding local variables
3. Making it vector-safe

This is still guarded by a ConstantInt check, so no functional change is intended.
But this should be ready to go: if we move the ConstantInt check down, all of
these folds should do the right thing for vector types.

llvm-svn: 279150