Fix for issue 26243 - illegal motion of assignments in instruction merging

tests/compiler/dart2js_extra/26243_test.dart

+import 'package:expect/expect.dart';
+
+var trace = [];
+void write(String s, int a, int b) {
+  trace.add("$s $a $b");
+}
+
+void foo() {
+  var i = 0;
+  write("foo", i, i += 1);
+}
+
+
+void bar() {
+  var i = 0;
+  try {
+    write("bar", i, i += 1);
+  } catch (_) {}
+}
+
+void baz() {
+  var i = 0;
+  write("baz-notry", i, i += 1);
+  
+  i = 0;
+  try {
+    write("baz-try", i, i += 1);
+  } catch (_) {}
+}
+
+void main() {
+  foo();
+  bar();
+  baz();
+  Expect.listEquals(['foo 0 1', 'bar 0 1', 'baz-notry 0 1', 'baz-try 0 1'],
+      trace);
+}