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.
The instruction parameter of CanMakeSynonymOf is an input parameter
that should never be null, so a const reference is a more appropriate
type than a const pointer.
The fuzzer pass was passing the type of a scalar where a vector type
was required, and was not checking whether synonyms could be made for
the operands to the scalar instruction.
Adds a new transformation that rewrites a scalar operation (like
OpFAdd, opISub) as an equivalent vector operation, adding a synonym
between the scalar result and an appropriate component of the vector
result.
Fixes#4195.
