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[x86] add cost overrides for mul with overflow

Authored by spatel on Oct 30 2020, 9:05 AM.

Description

[x86] add cost overrides for mul with overflow

I'm assuming the standard size integer instructions for this end up as something like:
mulq %rsi
seto %al

And the 'mul' generally has reciprocal throughput of 1 on typical implementations
(higher latency, but that's not handled here).
The default costs may end up much higher than that, and that's what we see in the test diffs.

Vector types are left as a 'TODO'.

Differential Revision: https://reviews.llvm.org/D90431

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