[LoongArch] Add basic support to AsmParser

This patch adds basic support to AsmParser which can handle basic
instructions with register or immediate operands. With the addition of
the parser, now it's possible to test instructions encoding with `llvm-mc`.

Disassembler will be added later and then we can do `round-trip` test.

Reviewed By: xen0n, MaskRay, myhsu

Differential Revision: https://reviews.llvm.org/D120476