[LoopUnswitch] Check OptimizeForSize before traversing over all basic blocks in curre...

[LoopUnswitch] Check OptimizeForSize before traversing over all basic blocks in current loop

Summary: This patch moves the check of OptimizeForSize before traversing over all basic blocks in current loop. If OptimizeForSize is set to true, no non-trivial unswitch is ever allowed. Therefore, the early exit will help reduce compilation time. This patch should be NFC.

Reviewers: reames, weimingz, broune

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11997

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244868 91177308-0d34-0410-b5e6-96231b3b80d8