[llvm] [RISCV] Remove vmv.s.x and vmv.x.s lmul pseudo variants (PR #71501)

[Luke Lau via llvm-commits]

lukel97 wrote:

> What would it take to remove the INSERT_SUBREG/EXTRACT_SUBREG from tblgen? Can we avoid creating the nodes with non-LMUL types in the first place?

I think we might be able to wrap all the places where we emit VMV_S_X_VL/VMV_X_S_VL in RISCVISelLowering with INSERT_SUBVECTOR/EXTRACT_SUBVECTORs instead. I'll give that a try

https://github.com/llvm/llvm-project/pull/71501