Use same range info when emitting code and computing if instruction can deopt.

runtime/tests/vm/dart/regress29620_test.dart

+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for dartbug.com/29620: check that decision to deoptimize
+// and decisions which parts of the instruction to emit use the same
+// range information for instruction inputs.
+
+// VMOptions=--enable-inlining-annotations --optimization_counter_threshold=10 --no-use-osr --no-background-compilation
+
+import "package:expect/expect.dart";
+
+const alwaysInline = "AlwaysInline";
+const neverInline = "NeverInline";
+
+class Flag {
+  var value;
+  Flag(this.value);
+
+  static final FLAG = new Flag(0);
+}
+
+@alwaysInline
+void checkRange(bit) {
+  if (bit < 0 || bit > 31) {
+    throw "bit must be in [0, 31]";
+  }
+}
+
+@alwaysInline
+bool isSet(flags, bit) {
+  checkRange(bit);
+  // Note: > 0 here instead of == 0 to prevent merging into
+  // TestSmi instruction.
+  return (flags & (1 << bit)) > 0;
+}
+
+@neverInline
+bool bug(flags) {
+  var bit = Flag.FLAG.value;
+  checkRange(bit);
+  for (var i = 0; i < 1; i++) {
+    bit = Flag.FLAG.value;
+    checkRange(bit);
+  }
+
+  // In early optimization stages `bit` would be a Phi(...). This Phi would be
+  // dominated by checkRange and thus range analysis will infer [0, 31] range
+  // for it - and thus a EliminateEnvironment will make decision that
+  // (1 << bit) can't deoptimize and will detach environment from it. Later
+  // passes will eliminate Phi for `bit` as it is redundant and as a result we
+  // will loose precise range information for `bit` and backend will try
+  // to emit a range check and a deoptimization.
+  return isSet(flags, bit);
+}
+
+main() {
+  for (var i = 0; i < 100; i++) {
+    bug(1);
+  }
+}