[IR] add optional parameter for copying IR flags to compare instructions

As shown, this is used to eliminate redundant code in InstCombine,
and there are more cases where we should be using this pattern, but
we're currently unintentionally dropping flags. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346282 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed