x86: add missing IgnoreSize

x86: add missing IgnoreSize

For proper code generation in 16-bit mode (or to avoid the "same type of
prefix used twice" diagnostic there), IgnoreSize is needed on certain
templates allowing for just 32-(and maybe 64-)bit operands.

Beyond adding tests for the previously broken cases, also add ones for
the previously working cases where IgnoreSize is needed for the same
reason (leaving out MPX for now, as that'll require an assembler change
first). Some minor adjustments to tests get done such that re-use of the
same code for 16-bit code generation testing becomes easier.