x86: operand sizing prefixes can disambiguate insns

x86: operand sizing prefixes can disambiguate insns

Use of an explicit data size or REX.W prefix is sufficient indication of
the intended operation when operand size can't be derived from suffix or
register operands. Avoid the ambiguity warning and make in particular
immediate handling (sizing) cope with explicitly specified prefixes.

Extending/reusing the noreg16 test made me notice a few cases of
unintentional 32-bit addressing, which gets corrected at the same time.