[SimpleLoopUnswitch] Early check exit for trivial unswitch with MemorySSA.

Summary:
If MemorySSA is avaiable, we can skip checking all instructions if block has any Defs.
(volatile loads are also Defs).
We still need to check all instructions for "canThrow", even if no Defs are found.

Reviewers: chandlerc

Subscribers: sanjoy, jlebar, Prazek, george.burgess.iv, llvm-commits

Differential Revision: https://reviews.llvm.org/D57129

llvm-svn: 352393