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[RISCV] Select (srl (sext_inreg X, i32), uimm5) to SRAIW if only lower 32 bits are used.

Authored by craig.topper on Sep 2 2021, 8:54 AM.



SimplifyDemandedBits can turn srl into sra if the bits being shifted
in aren't demanded. This patch can recover the original sra in some cases.

I've renamed the tablegen class for detecting W users since the "overflowing operator"
term I originally borrowed from Operator.h does not include srl.

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