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[RISCV] Improve lowering of SHL_PARTS/SRL_PARTS/SRA_PARTS.
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Authored by craig.topper on Feb 9 2022, 11:33 PM.

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Summary

Part of the shift lowering creates a (sub XLEN-1, ShAmt). When this
value is used we know that ShAmt is [0..XLEN-1]. Since XLEN is a power
of 2 we can replace the sub with an xor. This allows us to use XORI
instead of LI+SUB.

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