[llvm] [RISCV] Lower (vector_interleave X, undef) to (vzext_vl X). (PR #87283)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

And maybe ` (vector_interleave undef, X)` to `(vsll (vzext_vl X) sew_of_x)` for a separate patch?

https://github.com/llvm/llvm-project/pull/87283