x64: port icmp to ISLE

[abrown]

This change ports the lowering of the icmp instruction to ISLE.

[abrown]

@cfallin or @fitzgen: trying to port these instructions over is painful. I'm stuck on how to best lower this CMP + SETcc + SETcc sequence which involves new ways to produce and consume flags. That is what remains of the i128 lowering; the vector lowering should not be difficult.

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[fitzgen]

which involves new ways to produce and consume flags

Can this not use ProducesFlagsSideEffect and ConsumesFlagsTwiceReturnsValueRegs? The value regs will be invalid, but I think that should be fine.

[cfallin]

This looks good to me, thanks! Just one potential isel improvement idea below but nothing needed for correctness.

[cfallin]

We have support for emitting test I think, which IIRC is to and what cmp is to sub (i.e. doesn't write a dest, so wouldn't need to add to register pressure here) -- would it be possible to use that here?

[abrown]

Ah, yeah, that's a good point; let me do that in a follow-on PR.

[alexcrichton]

I haven't looked much into the cause, but fuzzers over the weekend found this bug in this PR -- #3951