[Loop Unswitch] Patch to selective unswitch only the reachable branch instructions.

Summary:
The iterative algorithm for Loop Unswitching may render some of the branches unreachable in the unswitched loops.
Given the exponential nature of the algorithm, this is quite an overhead.
This patch fixes this problem by selectively unswitching only those branches within a loop that are reachable from the loop header.

Reviewers: Michael Zolothukin, Anna Thomas, Weiming Zhao.
Subscribers: llvm-commits.

Differential Revision: http://reviews.llvm.org/D26299

llvm-svn: 287925