[LoongArch] AsmParser support for the li.[wd] pseudo instructions

The `li.[wd]` pseudo instructions are used to load an immediate value
into a GPR. These expand directly during asm parsing. As the result,
only real MC instructions are emitted to the MCStreamer. The actual
expansion to real instructions is similar to the expansion performed by
the GAS.

Note: The `li.w` always treats the imm operand as a 32-bit signed value.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D138086