[Issue 12832] asm movdqu accepts wrong operand size

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12832

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
What's happening is in iasm.d the operand size is encoded into 5 bit flags, and
there isn't room for another _128 bit, so the assembler just accepts any size.

The solution is to refactor the way operand sizes are encoded to accept more
values.

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