back.rtlil: Opportunistically trim zero and sign extension on operands.

[wanda-phi]

Fixes #1148.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.84%. Comparing base (2d59242) to head (f5f3d20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1152      +/-   ##
==========================================
+ Coverage   90.79%   90.84%   +0.04%     
==========================================
  Files          44       44              
  Lines       10519    10571      +52     
  Branches     2561     2577      +16     
==========================================
+ Hits         9551     9603      +52     
  Misses        777      777              
  Partials      191      191              

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[wanda-phi]

This is a little cursed and I'm worried about the potential for bugs, but we have to start doing randomized testing either way...

I would be much happier with at least a few tests, I think.

Yeah....

I do want to add some tests for this, but unfortunately this'll have to wait until #1100 lands, we don't really have good infrastructure for that right now.

[whitequark]

Since it's addressing a regresstion I'm not going to block this on #1100, @wanda-phi, your call on when to merge this.

[whitequark]

Oh, just noticed the branch name :D