[llvm] [RISCV] Correct the immediate swizzling for P-ext plui.h/w. (PR #149945)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

LGTM. This matches my reading, but all the docs are fairly information-thin on how the assembly corresponds to what is encoded.

https://github.com/llvm/llvm-project/pull/149945