Add comments and an assert to follow-up on r279113.  NFC.

Philip commented on r279113 to ask for better comments as to
when to use the different versions of getName.  Its also possible
to assert in the simple case that we aren't an overloaded intrinsic
as those have to use the more capable version of getName.

Thanks for the comments Philip.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279466 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed