[Verifier] Fix !dbg validation if Scope is the Subprogram

Summary:
We are inserting both Scope and SP into the Seen map and check whether
it was already there in which case we skip the validation (the idea
being that we already checked this Subprogram before). However,
if (Scope == SP) as MDNodes, then inserting the Scope, will trigger
the Seen check causing us to incorrectly not validate this !dbg
attachment. Fix this by not performing the SP Seen check if Scope == SP

Reviewers: pcc, dexonsmith, dblaikie

Subscribers: dblaikie, llvm-commits

Differential Revision: http://reviews.llvm.org/D14697

llvm-svn: 254887