[InstCombine] add helper for icmp with dominator; NFC

There's a potential small enhancement to this code that could
solve the cases currently under proposal in D54827 via SimplifyCFG.

Whether instcombine should be doing this kind of semi-non-local
analysis in the first place is an open question, but separating
the logic out can only help if/when we decide to move it to a
different pass. 

AFAICT, any proposal to do this in SimplifyCFG could also be seen 
as an overreach + it would be incomplete to start the fold from a 
branch rather than an icmp.

There's another question here about the code for processUGT_ADDCST_ADD().
That part may be completely dead after rL234638 ?

llvm-svn: 348273