Reland "[LoopUnswitch] Fix incorrect Modified status"

Relanded since the buildbot issue was unrelated to this commit.

When hoisting simple values out from a loop, and an optsize attribute, a
convergent call, or an invoke instruction hindered the pass from
unswitching the loop, the pass would return an incorrect Modified
status.

This was caught using the check introduced by D80916.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D86085