[LoongArch] Add definition for LSX instructions

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.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D154183