[NFC] Factor out implicit control flow logic from GVN

Logic for tracking implicit control flow instructions was added to GVN to
perform PRE optimizations correctly. It appears that GVN is not the only
optimization that sometimes does PRE, so this logic is required in other
places (such as Jump Threading).

This is an NFC patch that encapsulates all ICF-related logic in a dedicated
utility class separated from GVN.

Differential Revision: https://reviews.llvm.org/D40293

llvm-svn: 339086