Make PriorityMux stack safe (backport #2854)

[mergify]

This is an automatic backport of pull request #2854 done by Mergify.
Cherry-pick of 269ce47 has failed:

On branch mergify/bp/3.5.x/pr-2854
Your branch is up to date with 'origin/3.5.x'.

You are currently cherry-picking commit 269ce472.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   core/src/main/scala/chisel3/SeqUtils.scala
	new file:   src/test/scala/chiselTests/util/PriorityMuxSpec.scala

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   src/test/scala/chiselTests/util/PipeSpec.scala

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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