[TableGen][X86] Add Size field to X86MemOperand class

Set Size appropriately in operand definitions and query it for dumping memory
operand size table `getMemOperandSize` (follow-up use D126116) and
`X86Disassembler::getMemOperandSize`.

Excerpt from a produced `getMemOperandSize` table for X86:

```
static int getMemOperandSize(int OpType) {
  switch (OpType) {
  default: return 0;
  case OpTypes::i8mem:
  case OpTypes::i8mem_NOREX:
    return 8;

  case OpTypes::f16mem:
  case OpTypes::i16mem:
    return 16;

  case OpTypes::f32mem:
  case OpTypes::i32mem:
    return 32;
...
```

Reviewed By: skan, pengfei

Differential Revision: https://reviews.llvm.org/D127787