8229495: SIGILL in C2 generated OSR compilation

Reviewed-by: kvn, chagedorn

src/hotspot/share/opto/compile.cpp

@@ -1807,7 +1807,17 @@ void Compile::remove_opaque4_nodes(PhaseIterGVN &igvn) {
   for (int i = opaque4_count(); i > 0; i--) {
     Node* opaq = opaque4_node(i-1);
     assert(opaq->Opcode() == Op_Opaque4, "Opaque4 only");
+    // With Opaque4 nodes, the expectation is that the test of input 1
+    // is always equal to the constant value of input 2. So we can
+    // remove the Opaque4 and replace it by input 2. In debug builds,
+    // leave the non constant test in instead to sanity check that it
+    // never fails (if it does, that subgraph was constructed so, at
+    // runtime, a Halt node is executed).
+#ifdef ASSERT
+    igvn.replace_node(opaq, opaq->in(1));
+#else
     igvn.replace_node(opaq, opaq->in(2));
+#endif
   }
   assert(opaque4_count() == 0, "should be empty");
 }