Further cleanup and simplication of VFABI tests.

`tryDemangleForVFABI` and `VFShape`'s `getScalarShape`, `get` use
`FunctionType` instead of `CallInst`.

Each successful parsing should provide a vector name, and do checks for
the names, number of arguments, and vector parameter types, eg:

```
EXPECT_EQ(ScalarName, "foo");
EXPECT_EQ(VectorName, "vector_foo");
...
EXPECT_TRUE(matchScalarParamNum());
...
EXPECT_EQ(VecFuncParameters[0], VFParameter({0, VFParamKind::OMP_Linear, 2}));
```

Replace method names to `foo` and `vector_foo` for the scalar and vector
variants respectively.
Using only `i32` for parameters when it's not relevant, except when
the VF Elements are explicitly checked, `ptr` for references, and `void`
for most return types.

All `VFABIParserTest` tests are now listed contiguously in the source.

Author: paschalis-mpeis