[WinEH] Fix catchpad pred verification

Summary:
The code was simply ensuring that the catchpad's pred is its catchswitch,
which was letting cases slip through where the flow edge was the unwind
edge of the catchswitch rather than one of its catch clauses.

Reviewers: andrew.w.kaylor, rnk, majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16011

llvm-svn: 257275