[llvm] [RISCV][MC] Create an AsmOperand for carry-in vmask (PR #124317)

[Sam Elliott via llvm-commits]

lenary wrote:

@mshockwave is there a reason that this had to have a dummy parse/decode of a 0-bit "field", rather than, say, a `let Uses = [V0]` on the instructions? Does llvm-mca not pay attention to implicit uses?

https://github.com/llvm/llvm-project/pull/124317