[llvm] [RISCV] Use vmv.v.x to materialize masks in deinterleave2 lowering (PR #118500)

[Philip Reames via llvm-commits]

preames wrote:

> Can vmv.v.x be rematerialized? I thought only vmv.v.i can?

It can, see isReallyTriviallyReMaterializable.  During vector regalloc, the scalar operand can be ignored.  During scalar regalloc, the vector is already fixed.  

https://github.com/llvm/llvm-project/pull/118500