[X86] Use correct padding when in 16-bit mode
In 16-bit mode, some of the nop patterns used in 32-bit mode can end up
mangling other instructions. For instance, an aligned "movz" instruction
may have the 0x66 and 0x67 prefixes omitted, because the nop that's used
messes things up.
xorl %ebx, %ebx .p2align 4, 0x90 movzbl (%esi,%ebx), %ecx
Use instead nop patterns we know 16-bit mode can handle.
Differential Revision: https://reviews.llvm.org/D97268