[X86][Disassembler] Fix displacement operand size for symbolizer

On 64-bit X86, 0x66 operand-size override prefix will change the size of
the instruction operand, e.g. from 32 bits to 16 bits, but it will not
modify the size of the displacement operand used for memory addressing,
which will always be 32 bits.

Reviewed By: skan, rafauler

Differential Revision: https://reviews.llvm.org/D126726