Fix SInt literals to reject too small of widths (backport #4786)

[chiselbot]

Found by @tymcauley ucb-bar/fixedpoint#11 (comment)

This bug was introduced 2 years ago when we added support for zero-width literals: #2932

The problem is that BigInt.bitlength excludes the sign bit, so we need to add 1 to it for SInts (which is what we originally did), BUT we need special case support for zero-width SInts (only when specified).

Contributor Checklist

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Type of Improvement

• Bugfix

Desired Merge Strategy

• Squash

Release Notes

For users attempting to reinterpret raw 2s complement values as SInts, use .U.asSInt.

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---

This is an automatic backport of pull request #4786 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of db93161 has failed:

On branch mergify/bp/6.x/pr-4786
Your branch is up to date with 'origin/6.x'.

You are currently cherry-picking commit db931617.
  (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/internal/firrtl/IR.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

[jackkoenig]

Actually I don't want this backported. While it is technically a bug, there is code relying on this behavior that currently works a la ucb-bar/fixedpoint#11 (comment).