Fix cat of zero-width SInt

[jackkoenig]

Previously, concatenating two SInts where one is of zero-width would
return the non-zero-width SInt. This is incorrect because the output of
Cat should be of type UInt. Now the ZeroWidth transform will introduce a
cast when removing a Cat when the argument type is non-UInt.

Fixes #2096

h/t @drom for finding the issue and h/t @seldridge for identifying the cause.

For the input in #2096 we don't actually emit the expected behavior because we also constant propagate the register, so the result is:

module top_mod(
  input         clock,
  input         reset,
  input  [10:0] inp_e,
  output        tmp15
);
  assign tmp15 = 1'h0;
endmodule

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Type of Improvement

• bug fix

API Impact

No impact

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Fix bug in zero-width handling when the type of an SInt concatenated with a zero-width SInt could be incorrectly interpreted as an SInt (instead of a UInt) resulting in incorrect constant propagation.

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