[InstCombine] re-commit r218721 icmp-select-icmp optimization

Takes care of the assert that caused build fails.
Rather than asserting the code checks now that the definition
and use are in the same block, and does not attempt
to optimize when that is not the case.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219175 91177308-0d34-0410-b5e6-96231b3b80d8