Remove a bit of duplicated code.

Exactly the same checks are present in areTypesIsomorphic.

This might have been a premature performance optimization. I cannot reproduce
any slowdown with this patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222758 91177308-0d34-0410-b5e6-96231b3b80d8