assemble: defer "operand size missing" until end of type check

assemble: defer "operand size missing" until end of type check

Defer the "operand size missing" error until we know all the other
operands have the correct type.  Otherwise we'll end up with false
positives, which result in noise entered into the xsizeflags array,
thus causing fuzzy matching to fail.

It's possible we should defer it even further.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>