v2:
 - Adapted unit tests to make them consistent with the changes done
   to the validation of half-float conversions.

v3 (Curro):
- Check all the accummulators
- Constify declarations
- Do not check src1 type in single-source instructions.
- Check for all instructions that read accumulator (either implicitly or
  explicitly)
- Check restrictions in src1 too.
- Merge conditional block
- Add invalid test case.

v4 (Curro):
- Assert on 3-src instructions, as they are not validated.
- Get rid of types_are_mixed_float(), as we know instruction is mixed
  float at that point.
- Remove conditions from not verified case.
- Fix brackets on conditional.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>