nir: Add sample_positions_pan intrinsic

Facilites the gl_SamplePosition lowering on Bifrost, where the sample
positions are accessed directly in a packed in-memory format.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8774>

src/compiler/nir/nir_intrinsics.py

@@ -1101,6 +1101,9 @@ load("raw_output_pan", [1], [BASE], [CAN_ELIMINATE, CAN_REORDER])
 # src[] = { sampler_index }
 load("sampler_lod_parameters_pan", [1], flags=[CAN_ELIMINATE, CAN_REORDER])
 
+# Loads the sample position array on Bifrost, in a packed Arm-specific format
+system_value("sample_positions_pan", 1, bit_sizes=[64])
+
 # R600 specific instrincs
 #
 # location where the tesselation data is stored in LDS