[X86] Fix disassembly of x86-64 GDTLS code sequence

For x86-64 the REX.w prefix takes precedence over any other size
override (i.e. 0x66). Therefore, for x86-64 when REX.w is present set
'hasOpSize' to false to ensure that any size override is ignored.

Fixes PR48901.

Differential Revision: https://reviews.llvm.org/D95682