nir: Add a pass for selectively lowering variables to scratch space

This commit adds new nir_load/store_scratch opcodes which read and write
a virtual scratch space.  It's up to the back-end to figure out what to
do with it and where to put the actual scratch data.

v2: Drop const_index comments (by anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>

src/compiler/nir/meson.build

@@ -143,6 +143,7 @@ files_libnir = files(
   'nir_lower_phis_to_scalar.c',
   'nir_lower_regs_to_ssa.c',
   'nir_lower_returns.c',
+  'nir_lower_scratch.c',
   'nir_lower_subgroups.c',
   'nir_lower_system_values.c',
   'nir_lower_tex.c',