Make PriorityMux stack safe (backport #2854) (#2855)

* Make PriorityMux stack safe (#2854)

It used to be implemented with recursion, now it's implemented with a
stack safe reverse and foldLeft.

Also there were no tests for PriorityMux so I added one which helps
prove the change is functionally correct.

(cherry picked from commit 269ce472

)

# Conflicts:
#	src/test/scala/chiselTests/util/PipeSpec.scala

* Resolve backport conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>