broadcom/compiler: document that spill_base is used for spills and scratch

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15041>

src/broadcom/compiler/v3d_compiler.h

@@ -797,9 +797,11 @@ struct v3d_compile {
 
         /**
          * Register spilling's per-thread base address, shared between each
-         * spill/fill's addressing calculations.
+         * spill/fill's addressing calculations (also used for scratch
+         * access).
          */
         struct qreg spill_base;
+
         /* Bit vector of which temps may be spilled */
         BITSET_WORD *spillable;