SDC: Perform buffer propagation in two steps

Signed-off-by: Tomasz Michalak <tmichalak@antmicro.com>

sdc-plugin/clocks.cc

diff --git a/sdc-plugin/clocks.cc b/sdc-plugin/clocks.cc
index 85de7a6..82944a4 100644
--- a/sdc-plugin/clocks.cc
+++ b/sdc-plugin/clocks.cc
@@ -112,6 +112,11 @@
 	log("Processing clock %s\n", clock.Name().c_str());
 #endif
 	PropagateThroughBuffer(pass, clock, IBuf());
+    }
+    for (auto clock : clocks_) {
+#ifdef SDC_DEBUG
+	log("Processing clock %s\n", clock.Name().c_str());
+#endif
 	PropagateThroughBuffer(pass, clock, Bufg());
     }
 #ifdef SDC_DEBUG