[SimplifyCFG] Teach merge conditional stores to handle cases where the PostBB...

[SimplifyCFG] Teach merge conditional stores to handle cases where the PostBB has more than 2 predecessors by inserting a new block for the store.

Summary:
Currently merge conditional stores can't handle cases where PostBB (the block we need to move the store to) has more than 2 predecessors.

This patch removes that restriction by creating a new block with only the 2 predecessors we care about and an unconditional branch to the original block. This provides a place to put the store.

Reviewers: efriedma, jmolloy, ABataev

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39760

llvm-svn: 329142