[InstCombine] Re-commit of  r218721 (Optimize icmp-select-icmp sequence)

Fixes the self-host fail. Note that this commit activates dominator
analysis in the combiner by default (like the original commit did).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222590 91177308-0d34-0410-b5e6-96231b3b80d8