[SimpleLoopUnswitch] Update DefaultExit condition to check unreachable is not empty.

Summary:
Update the check for the default exit block to not only check that the
terminator is not unreachable, but also check that unreachable block has
*only* the unreachable instruction.

Reviewers: chandlerc

Subscribers: hiraditya, uabelho, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78277