x86: replace adhoc (partly wrong) ambiguous operand checking for MOVSX/MOVZX

For these to get treatment consistent with other operand size checking
the special logic shouldn't live in md_assemble(), but process_suffix().
And there's more logic involved than simply zapping the suffix.

Note however that MOVS[BW]* and MOVZ[BW]* still won't be fully
consistent, due to the objection to fold MOVS* templates just like was
done for MOVZ* in c07315e0c6 ("x86: allow suffix-less movzw and 64-bit
movzb").

Note further that it is against my own intentions to have MOVSX/MOVZX
silently default to a byte source in AT&T mode. This should happen only
when the destination register is a 16-bit one. In all other cases there
is an ambiguity, and the user should be warned. But it was explicitly
requested for this to be done in a way inconsistent with everything
else.

Note finally that the assembler change points out (and this patch fixes)
a wrong Intel syntax test introduced by bc31405ebb2c ("x86-64: Properly
encode and decode movsxd"): When source code specifies a 16-bit
destination register, disassembly expectations shouldn't have been to
find a 32-bit one.