[LoongArch] Support parsing la* pseudo instructions

This patch makes `IAS` compatible with `GAS`. It accepts `la*` pseudo
instructions, and expands `la{,.local,.global}` into different
instructions according to different features.

```
 Default:
     la = la.global = la.got
     la.local = la.pcrel
 With feature "+la-global-with-pcrel":
     la = la.global = la.pcrel
 With feature "+la-global-with-abs":
     la = la.global = la.abs
 With feature "+la-local-with-abs":
     la.local = la.abs
 With features "+la-global-with-pcrel,+la-global-with-abs"(disorder):
     la = la.global = la.pcrel
```
Note: To keep consistent with `GAS` behavior, the "la" can only have
      one register operand.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D138021