[TLI] Unify LibFunc signature checking. NFCI.

I tried to be as close as possible to the strongest check that
existed before; cleaning these up properly is left for future work.

Differential Revision: http://reviews.llvm.org/D19469

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267758 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed