[llvm] [RISCV][P-ext] Add mvd alias for padd.dw rd, zero, rs. Use for copy idiom. (PR #205223)

[Sam Elliott via llvm-commits]

lenary wrote:

> We should probably also update `copyPhysReg` as it puts x0 in the wrong operand of the `padd.dw`

Nvm, that's in these changes. 

https://github.com/llvm/llvm-project/pull/205223