[NewPM][LoopUnswitch] Pin loop-unswitch to legacy PM or use simple-loop-unswitch

As mentioned in
http://lists.llvm.org/pipermail/llvm-dev/2020-July/143395.html,
loop-unswitch has not been ported to the NPM. Instead people are using
simple-loop-unswitch.

Pin all tests in Transforms/LoopUnswitch to legacy PM and replace all
other uses of loop-unswitch with simple-loop-unswitch.

One test that didn't fit into the above was
2014-06-21-congruent-constant.ll which seems to only pass with
loop-unswitch. That is also pinned to legacy PM.

Now all tests containing "-loop-unswitch" anywhere in the test succeed with
NPM turned on by default.

Reviewed By: ychen

Differential Revision: https://reviews.llvm.org/D85360