[InstCombine] rearrange code for foldSelectBinOpIdentity; NFCI

This is a retry of rL339439 with a fix for the problem that
caused the original commit to be reverted at rL339446. 

That problem was that 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.

A test to guard against the problem was added at rL339453.

llvm-svn: 339469