[LoongArch] Add EncoderMethods for transformed immediate operands

This is a split patch of D120476 and thanks to myhsu.

'Transformed' means the encoding of an immediate is not the same as
its binary representation. For example, the `bl` instruction
requires a signed 28-bits integer as its operand and the low 2 bits
must be 0. So only the upper 26 bits are needed to get encoded into
the instruction.

Based on the above reason this kind of immediate needs a customed
`EncoderMethod` to get the real value getting encoded into the
instruction.

Currently these immediate includes:
```
  uimm2_plus1
  simm14_lsl2
  simm16_lsl2
  simm21_lsl2
  simm26_lsl2
```

This patch adds those `EncoderMethod`s and revises related .mir test
in previous patch.

Reviewed By: xen0n, MaskRay

Differential Revision: https://reviews.llvm.org/D120545