Recognize pattern (a << b) & c with c being a positive Smi and allow left shift to truncate the res…

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 18705)
+++ runtime/vm/compiler.cc	(working copy)
@@ -220,6 +220,10 @@
         optimizer.Canonicalize();
       }
 
+      // Optimize (a << b) & c patterns. Must occur before
+      // 'Selectrepresentations' which inserts conversion nodes.
+      optimizer.TryOptimizeLeftShiftWithBitAndPattern();
+
       // Unbox doubles. Performed after constant propagation to minimize
       // interference from phis merging double values and tagged
       // values comming from dead paths.