[SimpleLoopUnswitch] Unswitch by experimental.guard intrinsics

This patch adds support of `llvm.experimental.guard` intrinsics to non-trivial
simple loop unswitching. These intrinsics represent implicit control flow which
has pretty much the same semantics as usual conditional branches. The
algorithm of dealing with them is following:

- Consider guards as unswitching candidates;
- If a guard is considered the best candidate, turn it into a branch;
- Apply normal unswitching algorithm on this branch.

The patch has no compile time effect on code that does not contain any guards.

Differential Revision: https://reviews.llvm.org/D53744
Reviewed By: chandlerc

llvm-svn: 345387