ra: Document that class index is allocated in order, use that in r300.

etnaviv also relies on this being the case, just drop the remapping.

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9437>

src/gallium/drivers/r300/compiler/radeon_pair_regalloc.c

@@ -722,8 +722,7 @@ void rc_init_regalloc_state(struct rc_regalloc_state *s)
 	for (i = 0; i < RC_REG_CLASS_COUNT; i++) {
 		ra_q_values[i] = MALLOC(RC_REG_CLASS_COUNT * sizeof(unsigned));
 		for (j = 0; j < RC_REG_CLASS_COUNT; j++) {
-			ra_q_values[s->class_ids[i]][s->class_ids[j]] =
-							q_values[i][j];
+			ra_q_values[i][j] = q_values[i][j];
 		}
 	}
 

src/util/register_allocate.c

@@ -227,6 +227,7 @@ ra_alloc_reg_class(struct ra_regs *regs)
 
    class->regs = rzalloc_array(class, BITSET_WORD, BITSET_WORDS(regs->count));
 
+   /* Users may rely on the class index being allocated in order starting from 0. */
    return regs->class_count++;
 }