[llvm] [RISCV][MC] Remove register substitutions in `RISCVAsmPrinter`. NFC (PR #102728)

[Craig Topper via llvm-commits]

topperc wrote:

I don't think it's a good idea to create MCInst with registers that don't match their register class declared in tablegen.

https://github.com/llvm/llvm-project/pull/102728