Makes the fuzzer pass and transformation that wraps vector synonyms
aware of the fact that integer operations can have arguments that
differ in signedness, and that the result type of such an operation
can have different sign from the argument types.
Fixes#4413.
Adaps the transformations that add OpConstantUndef and OpConstantNull
to a module so that pointer undefs are not allowed, and null pointers
are only allowed if suitable capabilities are present.
Fixes#4357.
