[PM/LoopUnswitch] Remove a buggy assert in the new loop unswitch.

The condition this was asserting doesn't actually hold. I've added
comments to explain why, removed the assert, and added a fun test case
reduced out of 403.gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330564 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed