[LoopUnswitch] Fix a simple bug which disables loop unswitch for select statement

This is to fix PR34257. rL309059 takes an early return when FindLIVLoopCondition
fails to find a loop invariant condition. This is wrong and it will disable loop
unswitch for select. The patch fixes the bug.

Differential Revision: https://reviews.llvm.org/D36985

llvm-svn: 312045