The hope here is to replace our backend handling for loading whole
cachelines at a time from UBOs into NIR-based handling, which plays
nicely with the NIR load/store vectorizer.
Rounding down offsets to multiples of 64B allows us to globally CSE
UBO loads across basic blocks. This is really useful. However, blindly
rounding down the offset to a multiple of 64B can trigger anti-patterns
where...a single unaligned memory load could have hit all the necessary
data, but rounding it down split it into two loads.
By moving this to NIR, we gain more control of the interplay between
nir_opt_load_store_vectorize and this rebasing and CSE'ing. The backend
can then simply load between nir_def_{first,last}_component_read() and
trust that our NIR has the loads blockified appropriately.
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32888>
