[X86][MC] Support encoding optimization & assembler relaxation about...

[X86][MC] Support  encoding optimization & assembler relaxation about immediate operands for APX instructions (#78545)

Encoding optimization:
```
mi/mi32 -> mi8
ri/ri32 -> ri8
```
if the immediate operand is 8-bit wide.

Assembler relaxation:
```
mi8 -> mi/mi32
ri8 -> ri/ri32
```
If the immediate operand is a symbol expression and it's value is
unknown.