8258790: C2: Crash on empty macro node list

Reviewed-by: kvn, chagedorn

src/hotspot/share/opto/macro.cpp

@@ -2563,11 +2563,8 @@ void PhaseMacroExpand::eliminate_macro_nodes() {
   bool progress = true;
   while (progress) {
     progress = false;
-    for (int i = C->macro_count(); i > 0; i--) {
-      if (i > C->macro_count()) {
-        i = C->macro_count(); // more than 1 element can be eliminated at once
-      }
-      Node* n = C->macro_node(i-1);
+    for (int i = C->macro_count(); i > 0; i = MIN2(i - 1, C->macro_count())) { // more than 1 element can be eliminated at once
+      Node* n = C->macro_node(i - 1);
       bool success = false;
       DEBUG_ONLY(int old_macro_count = C->macro_count();)
       if (n->is_AbstractLock()) {
@@ -2582,11 +2579,8 @@ void PhaseMacroExpand::eliminate_macro_nodes() {
   progress = true;
   while (progress) {
     progress = false;
-    for (int i = C->macro_count(); i > 0; i--) {
-      if (i > C->macro_count()) {
-        i = C->macro_count(); // more than 1 element can be eliminated at once
-      }
-      Node* n = C->macro_node(i-1);
+    for (int i = C->macro_count(); i > 0; i = MIN2(i - 1, C->macro_count())) { // more than 1 element can be eliminated at once
+      Node* n = C->macro_node(i - 1);
       bool success = false;
       DEBUG_ONLY(int old_macro_count = C->macro_count();)
       switch (n->class_id()) {