Generalize the code over the properties of the mov instruction, rather than maintaining parallel logic to figure out the type of mov to use. I've maintained the behavior with 16-bit physical SGPRs, though I think the behavior here is broken and corrupting any value that happens to be live in the high bits. It just happens there's no way to separately write to those with a real instruction but I don't think we should be trying to make assumptions around that property. This is NFC-ish. It now does a better job with imm pseudos which practically won't reach here. This also will make it easier to support more folds in a future patch. I added a couple of new tests with 16-bit extract of 64-bit sources. The only other test change is an immediate rendering change from zero extended to sign extended.
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