spirv-fuzz: consider using VariablePointers capability

Some transformations would require using OpSelect to select between
two variables. However, this requires the VariablePointers capability.

Would adding this capability be sensible in some cases?

NOTE: This capability would also allow creating OpVariable
instructions that point to pointers. This might cause some
problems in the code in spirv-fuzz which assumes that pointers
always point to either a scalar or a composite.

Start by examining the spirv-fuzz transformations that would need OpSelect and the code assumptions around OpVariable pointer types. Determine whether enabling the VariablePointers capability is safe when pointers can point to pointers, and define the affected transformation behavior and tests before making a change.

Written by the indexing model from the issue text.