[InstCombine] add/update tests for selectBinOpIdentity; NFC

This includes a test that would have exposed the bug in rL339439
which was reverted at rL339446. The compare can be integer while
the binop is FP or vice-versa, so we need to use the binop type
when we ask for the identity constant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339453 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed