In some cases we need to bitcast when dealing with int vs. uint.
SPIR-V allows inputs to be of different integer signedness, so we need
to deal with this somehow.
Add testing system to test SPIR-V assembly.
For now, test all possible combination for all major cases.
- IAdd (which doesn't care about input type as long as they're equal)
- SDiv/UDiv operations which case about input type.
- Arith/Logical right shifts.
- IEqual to test outputs to bvec, which shouldn't get output cast. Also
tests casting in function-like calls.
