[InstCombine] Fix for assert build failures caused by r218721

The icmp-select-icmp optimization made the implicit assumption
that the select-icmp instructions are in the same block and asserted on it.
The fix explicitly checks for that condition and conservatively suppresses
the optimization when it is violated.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218735 91177308-0d34-0410-b5e6-96231b3b80d8