[InstCombine] Move the checks for pointer types in getMaskedTypeForICmpPair earlier in the function

I don't think there's any reason to have them scattered about and on all 4 operands. We already have an early check that both compares must be the same type. And within a given compare the LHS and RHS must have the same type. Beyond that I don't think there's anyway this function returns anything valid for pointer types. So let's just return early and be done with it.

Differential Revision: https://reviews.llvm.org/D36561

llvm-svn: 311383