intel/compiler: Add backend compiler basics for Task/Mesh

intel/compiler: Add backend compiler basics for Task/Mesh

Task/Mesh stages are CS-like stages, and include many
builtins (e.g. workgroup ID/index) and intrinsics (e.g. workgroup
memory primitives) originally present only in CS.

This commit add two new stages (task and mesh) that 'inherit' from CS
by embedding a brw_cs_prog_data in their own prog_data structure, so
that CS functionality can be easily reused.  They also currently use
the same helpers to select the SIMD variant to use -- that was
recently added for CS.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13661>