[llvm] [RISCV] Use ri.vunzip2{a,b} for e64 fixed length deinterleave(2) shuffles (PR #137217)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Is there a reason why this doesn't handle < e64? Is it that hardware expected to have similar performance with a single vnsrl.wx vs ri.vunzip and we want to prefer the standard extension?

https://github.com/llvm/llvm-project/pull/137217