Legalize neg: -x becomes 0 - x

[jackkoenig]

This fixes an error with negating a negative SInt literal and a
[debatable] lint warning in Verilator when negating any value.

This behavior matches that of Chisel (which directly emits the 0 - x
already).

Fixes #2026 although it introduces a new lint warning for that case
Fixes #2031

Follow on work to do constant propagation on subtraction (which we somehow are still not doing) which will fix the introduced lint warning (but a lint warning is better than an error!)

Contributor Checklist

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Type of Improvement

• bug fix
• backend code generation

API Impact

Depending on Legalize means a transform will no longer see Neg prim ops.

Backend Code Generation Impact

This changes the emission of neg from:

-$signed(x);

to

8'h0 - $signed(x);

As described in #2031, Verilator has a lint warning on this (warnings default to error). I don't agree with the lint warning, but since Chisel doesn't emit neg anyway (instead emitting 0 - x like this change), I think matching the behavior of Chisel is fine.

We should consider seeing if Verilator might change that warning though.

Desired Merge Strategy

• Squash

Release Notes

Canonicalize neg(x) to 0 - x. Fix bug where negated negative literal would result in invalid Verilog.

Reviewer Checklist (only modified by reviewer)

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