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/nir_clone.c

@@ -735,6 +735,7 @@ nir_shader_clone(void *mem_ctx, const nir_shader *s)
    ns->num_uniforms = s->num_uniforms;
    ns->num_outputs = s->num_outputs;
    ns->num_shared = s->num_shared;
+   ns->scratch_size = s->scratch_size;
 
    ns->constant_data_size = s->constant_data_size;
    if (s->constant_data_size > 0) {