[InstCombine] re-commit r218721 with fix for pr21199

The icmp-select-icmp optimization targets select-icmp.eq
only. This is now ensured by testing the branch predicate
explictly. This commit also includes the test case for pr21199.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219282 91177308-0d34-0410-b5e6-96231b3b80d8