Reapply "Only emit typeguards if we think they are valuable."

lib/compiler/implementation/ssa/bailout.dart

Index: lib/compiler/implementation/ssa/bailout.dart
diff --git a/lib/compiler/implementation/ssa/bailout.dart b/lib/compiler/implementation/ssa/bailout.dart
index dcc64081ebc7df3f00dbfc8ae180aba2690a5e86..daf0e3018276fed4e59747d9d8d0f69fe2795e37 100644
--- a/lib/compiler/implementation/ssa/bailout.dart
+++ b/lib/compiler/implementation/ssa/bailout.dart
@@ -111,6 +111,27 @@ class SsaTypeGuardInserter extends HGraphVisitor implements OptimizationPhase {
     }
   }
 
+  bool typeGuardWouldBeValuable(HInstruction instruction,
+                                HType speculativeType) {
+    bool isNested(HBasicBlock inner, HBasicBlock outer) {
+      if (inner === outer) return false;
+      if (outer === null) return true;
+      while (inner !== null) {
+        if (inner === outer) return true;
+        inner = inner.parentLoopHeader;
+      }
+      return false;
+    }
+
+    // If the instruction is not in a loop then the header will be null.
+    HBasicBlock currentLoopHeader = instruction.block.enclosingLoopHeader;
+    for (HInstruction user in instruction.usedBy) {
+      HBasicBlock userLoopHeader = user.block.enclosingLoopHeader;
+      if (isNested(userLoopHeader, currentLoopHeader)) return true;
+    }
+    return false;
+  }
+
   bool shouldInsertTypeGuard(HInstruction instruction) {
     HType speculativeType = instruction.propagatedType;
     HType computedType = instruction.computeTypeFromInputTypes();
@@ -128,8 +149,9 @@ class SsaTypeGuardInserter extends HGraphVisitor implements OptimizationPhase {
     if (!speculativeType.isUseful()) return false;
     // If the types agree we don't need to check.
     if (speculativeType == computedType) return false;
-    // TODO(floitsch): Make the creation of type guards more conditional.
-    return true;
+    // If a bailout check is more expensive than doing the actual operation
+    // don't do it either.
+    return typeGuardWouldBeValuable(instruction, speculativeType);
   }
 
   void visitInstruction(HInstruction instruction) {