r600: ignore dest sel for non-write targets when counting registers

Since the value is not written, there is no need to allocate
a register for it, so don't take it into account.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15714>

src/gallium/drivers/r600/r600_asm.c

@@ -1302,7 +1302,7 @@ int r600_bytecode_add_alu_type(struct r600_bytecode *bc,
 			r600_bytecode_special_constants(nalu->src[i].value,
 				&nalu->src[i].sel);
 	}
-	if (nalu->dst.sel >= bc->ngpr) {
+	if (nalu->dst.write && nalu->dst.sel >= bc->ngpr) {
 		bc->ngpr = nalu->dst.sel + 1;
 	}
 	list_addtail(&nalu->list, &bc->cf_last->alu);