Use range analysis to improve constant propagation.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 20842)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -608,7 +608,8 @@
 BranchInstr::BranchInstr(ComparisonInstr* comparison, bool is_checked)
     : comparison_(comparison),
       is_checked_(is_checked),
-      constrained_type_(NULL) {
+      constrained_type_(NULL),
+      constant_target_(NULL) {
   for (intptr_t i = comparison->InputCount() - 1; i >= 0; --i) {
     comparison->InputAt(i)->set_instruction(this);
   }
@@ -2192,6 +2193,19 @@
 }
 
 
+bool Range::IsUnsatisfiable() const {
+  // Constant case: For example [0, -1].
+  if (Range::ConstantMin(this).value() > Range::ConstantMax(this).value()) {
+    return true;
+  }
+  // Symbol case: For example [v+1, v].
+  if (DependOnSameSymbol(min(), max()) && min().offset() > max().offset()) {
+    return true;
+  }
+  return false;
+}
+
+
 bool CheckArrayBoundInstr::IsFixedLengthArrayType(intptr_t cid) {
   return LoadFieldInstr::IsFixedLengthArrayCid(cid);
 }