[PM/LoopUnswitch] Avoid pointlessly creating an exit block set.

This code can just test whether blocks are *in* the loop, which we
already have a dedicated set tracking in the loop itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332004 91177308-0d34-0410-b5e6-96231b3b80d8