InstCombine: Fix a combine assuming that icmp operands were integers

An icmp may have pointer arguments, it isn't limited to integers or
vectors of integers.

This fixes PR21388.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220664 91177308-0d34-0410-b5e6-96231b3b80d8