dzn: Fix vertex input handling

dzn: Fix vertex input handling

Vertex shaders are allowed to define input variables pointing to the
same location but a different, or even variables that overlap other
variables, as long as only one of them is used in a shader invocation.

One way to support that case would be to merge overlapping variables,
but we can also declare one input element per variable, and make those
point to the same input slot/offset. The only limitation with the
second approach is the maximum number of VS input registers, meaning
that only (32 - num_sysvals) input variables can be defined.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16221>