Simplified createFunctionType method

It accepts ScalarFTy and VecRetTy. As the latter is not kept in VFABI,
it should instead come from the original Instruction/Callinst.

This change allows further simplification in tests. Also, methods like
the test `_ZGVnN3lLRUlnLnRnUn_foo`, which contains only scalar parameters,
would still return a valid FunctionType. This is an edge case that will
only be encountered in tests though.