intel/rt: Implement push constants as global memory reads

They're not really "push" anymore but that's because there is no such
thing as push constants in bindless shaders on Intel.  They should be
fast enough, though.  There is some room for debate here as to whether
we want to do the pull in NIR or push it into the back-end.  The
advantage of doing it in the back-end is that it'd be easier to use
MOV_INDIRECT for indirect push constant access rather than falling back
to a dataport message.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7356>

src/intel/compiler/brw_rt.h

@@ -31,6 +31,9 @@ extern "C" {
 /** Vulkan defines shaderGroupHandleSize = 32 */
 #define BRW_RT_SBT_HANDLE_SIZE 32
 
+/** Offset after the RT dispatch globals at which "push" constants live */
+#define BRW_RT_PUSH_CONST_OFFSET 128
+
 /** Stride of the resume SBT */
 #define BRW_BTD_RESUME_SBT_STRIDE 8