agx: Add a hash table for vector extracts

This will allow us to introduce splits gradually, giving a graceful fallback.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16268>

src/asahi/compiler/agx_compiler.h

@@ -411,6 +411,10 @@ typedef struct {
    agx_block *break_block;
    agx_block *after_block;
 
+   /* During instruction selection, map from vector agx_index to its scalar
+    * components, populated by a split. */
+   struct hash_table_u64 *allocated_vec;
+
    /* Stats for shader-db */
    unsigned loop_count;
    unsigned spills;