[SimplifyCFG] Fix the determination of PostBB in conditional store merging to...

[SimplifyCFG] Fix the determination of PostBB in conditional store merging to handle the targets on the second branch being commuted

Currently we choose PostBB as the single successor of QFB, but its possible that QTB's single successor is QFB which would make QFB the correct choice.

Differential Revision: https://reviews.llvm.org/D32323

llvm-svn: 300992