intel: Drop the last uses of a mem_ctx in nir_builder_init_simple_shader().

intel: Drop the last uses of a mem_ctx in nir_builder_init_simple_shader().

These two consumers were the only ones out of the ~65 calls to
init_simple_shader, so there's a pretty clear consensus on how to allocate
simple shaders.  I suspect that actually these would be just fine with
b.shader being the mem_ctx, but that would take a bit more rework.

Acked-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7323>