ice40_dsp: fix const handling

[Anhijkt]

What are the reasons/motivation for this change?
ice40_dsp seems to incorrectly handle unsigned consts. The issue seems to be with unextend function. By searching previous issues I could find this one #2648.
Explain how this is achieved.
Now unextend is called only on non constant inputs.
If applicable, please suggest to reviewers how they can test the change.

read_verilog << EOT
module top(input wire [14:0] a, output wire [31:0] b);
assign b = a*$unsigned(5'b01111);
endmodule
EOT

prep
ice40_dsp
write_rtlil

read_verilog << EOT
module top(input wire signed [14:0] a, output wire signed [31:0] b);
assign b = a*$signed(5'b01111);
endmodule
EOT

prep
ice40_dsp
write_rtlil

Gives two different connect \B values in \SB_MAC16 cell on pre-PR code, and the same value on post-PR code.

[widlarizer]

Please add a regression test. If you have the time, can you check techlibs/xilinx/xilinx_dsp.pmg too? It seems like either is the copy of the other, and they differ exactly on const handling. Maybe what xilinx_dsp.pmg does is somehow a more correct fix, or the other way around. Thanks for your contributions!

[Anhijkt]

I tried to copy xilinx code, but it(and microchip_dsp.pmg) seems to fail on the same issue. Also I mistaken this bug for const-only, turns out this is an issue with handling all unsigned signals (for example a*$unsigned({7'b1110101, b})), so I think there is other way to fix it.

[Anhijkt]

Also I want to note that in first example, the issue is not just with ice40_dsp, unextend can handle $unsigned(5'b01111), the problem is with wreduce in prep, that reducing it into $unsigned(4'b1111).

[Anhijkt]

The previous commit did not optimize unsigned zero-leading signals unlike the original code. But it also differs from original when handling signals like $unsigned({a[n], a[n], a}) where n is the index of last bit of signal a. If original unextend would optimize it into $unsigned({a}) this code leave it unchanged. I dont know which implementation is more accurate, so I think I need feedback on this one.