Rewrite the function-local validation logic for MDNodes (most of r91708).

Rewrite the function-local validation logic for MDNodes (most of r91708).
Among other benefits, this doesn't leak the SmallPtrSet, has the verifier
code in the verifier pass, actually does the verification at the end,
and is considerably simpler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92217 91177308-0d34-0410-b5e6-96231b3b80d8