nir: Always create UBO variable when lowering uniforms to ubo

nir: Always create UBO variable when lowering uniforms to ubo

Zink needs to know the sizes of UBOs, and for normal UBOs we get this
from the nir_var_mem_ubo variables. This allows us to treat all of these
the same way.

We're about to need the same information for the in-progress D3D12
driver, so let's do this in a central location instead of in the driver.

This version is also a bit more careful than the Zink version. In
particular, for two reasons:
1. We increase the variable bindings when we adjust the pre-existing
   UBOs.
2. We increase shader->info.num_ubos when we insert a new UBO variable.

Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4734>