r300/compiler: Don't track readers into an IF block.

This makes rc_get_readers_normal() more conservative than it needs to be,
but it fixes some incorrect behavior in the optimization passes.
diff --git a/src/mesa/drivers/dri/r300/compiler/radeon_dataflow.c b/src/mesa/drivers/dri/r300/compiler/radeon_dataflow.c
index 5927498..885e227 100644
--- a/src/mesa/drivers/dri/r300/compiler/radeon_dataflow.c
+++ b/src/mesa/drivers/dri/r300/compiler/radeon_dataflow.c
@@ -624,6 +624,9 @@
 			data->Abort = 1;
 			return;
 		case RC_OPCODE_IF:
+			/* XXX We can do better here, but this will have to
+			 * do until this dataflow analysis is more mature. */
+			data->Abort = 1;
 			branch_depth++;
 			break;
 		case RC_OPCODE_ELSE: