[RISCV] Combine GREVI sequences

[frasercrmck]

Summary

This combine step performs the following type of transformation:

rev.p a0, a0   # grevi a0, a0, 0b01
rev2.n a0, a0  # grevi a0, a0, 0b10
-->
rev.n a0, a0   # grevi a0, a0, 0b11

[craig.topper]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

1374

Does the order matter? Or do we just need (ShAmt1 & ShAmt2) == 0 to hold?

[craig.topper]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

1374

Maybe we don't even need (ShAmt1 & ShAmt2) == 0 if we XOR the constants. Maybe folding 0 to GREVSrc.

[frasercrmck]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

1374

You're right; the order isn't important. Not sure what I was thinking. XORing seems like the best way to go.

[frasercrmck]

removed ordering & simplified logic. added extra test.

[craig.topper]

LGTM