[SimpleLoopUnswitch] Make all checks before actual non-trivial unswitch

We should be able to make all relevant checks before we actually start the non-trivial
unswitching, so that we could guarantee that once we have started the transform,
it will always succeed.

Reviewed By: chandlerc
Differential Revision: https://reviews.llvm.org/D53747

llvm-svn: 345375