If we have emitted block IO lowering at the end of vertex shader, we
will end up using the wrong name. Forcing a v_ prefix does not solve any
actual problems since the intentifier already has to be valid.
On MSL, the compiler refuses to allow access chains into a normal vector type.
What happens in practice instead is a read-modify-write where a vector type is
loaded, modified and written back.
The workaround is to convert a vector into a pointer-to-scalar before
the access chain continues to add the scalar index.
