[Arc] Simulation specific optimization for comb.icmp

[maerhart]

On the HW/Comb level, the comb.concat operation is basically considered free. However, in simulation it is quite expensive. Instead of concatenating integers of the same width, we can do the AND/OR directly and do a reduction on the smaller result integer, effectively cutting the number of assembly instructions produced to almost half.

[fabianschuiki]

Clever! LGTM

[maerhart]

Some statistics with and without the ICMP pattern.

Design	Total Instr. w/o	Total Instr. w	%	Stack Instr. w/o	Stack Instr. w	%
Riscinator	6084	3235	53.2	1471	941	63.0
Rocket	127622	122454	96.0	33719	32627	96.8

Designs from github.com/circt/arc-tests
Stack Instr measured with: objdump -d design.o | grep -E "push|pop|rsp|rbp" | wc -l
Total Instr measured with objdump -d design.o | wc -l

[fabianschuiki]

Holy cow, I didn't expect this pattern to be that impactful. Very cool stuff!