nak: implement SHL, SHR, and IMUL on SM50
The SM75 behavior for SHF and IMAD64 cannot be implemented on SM50, so instead I added three SM50-specific instructions to replace the cases where they are currently used.
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The SM75 behavior for SHF and IMAD64 cannot be implemented on SM50, so instead I added three SM50-specific instructions to replace the cases where they are currently used.