[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

llvm-svn: 244868