Detailed description
I've confirmed #16433 (comment) (including the nasm rax -> eax bug) and thus this pr
makes the x86_64 assembler use the MOV B8+ encoding (MOV r64, imm64) when the immediate
is between 0x80000000 and 0xffffffff for both:
Consistency with GNU assembler. It actually does seem to be the expected encoding.
This does mean that mov eax, 0xffffffff and mov rax, 0xffffffff will have very different semantics.
