Implements integer dot product lowering both with and without
DP4A. Implements half-float dot product lowering.
There are a couple FINISHME comments describing future optimizations.
v2: Add a brw_compiler::lower_dpas flag to track when the lowering
should be applied.
v3: Use is_null() instead of checking file != ARF. Suggested by Caio.
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25994>
