This patch adds the definition for the LSX registers and instructions.
It also adds handling for new immediate operands in the AsmParser. This
patch ensures that llvm-mc and llvm-objdump correctly handle the LSX
instructions.
We expand those pseudo-instructions vrepli.{b,h,w,d} in the
MCCodeEmitter. This increases the readability of the output when
generating assembly files.
This may have to become generic when we add xvldi later, so I'd recommend adding a template argument for substituting the LoongArch::VLDI at call site. Doing so will also let the name make sense because there isn't a ldi per se. (You may also name this expandToVectorLDI or expandVectorREPLI instead for maximum clarity.)