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.
