Preserve literals across .asUInt (backport #4148)

[mergify]

Also working on supporting preserving literals across .asTypeOf, but this is the first step.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Feature (or new API)

Desired Merge Strategy

• Squash

Release Notes

This is mostly useful for initial values for async reset registers and for constructing literal values in testing contexts (e.g. ChiselSim). It also should slightly reduce memory use and .fir size.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

---

This is an automatic backport of pull request #4148 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d35daa2 has failed:

On branch mergify/bp/6.x/pr-4148
Your branch is up to date with 'origin/6.x'.

You are currently cherry-picking commit d35daa264.
  (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/Aggregate.scala
	modified:   core/src/main/scala/chisel3/Bits.scala
	modified:   src/test/scala/chiselTests/BundleLiteralSpec.scala
	modified:   src/test/scala/chiselTests/UIntOps.scala
	modified:   src/test/scala/chiselTests/VecLiteralSpec.scala

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/test/scala/chiselTests/SIntOps.scala

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