Removes undefined shifting behavior from BitVector::Equals.

runtime/vm/bit_vector.cc

Index: runtime/vm/bit_vector.cc
===================================================================
--- runtime/vm/bit_vector.cc	(revision 24832)
+++ runtime/vm/bit_vector.cc	(working copy)
@@ -41,8 +41,8 @@
   }
   if (i < data_length_) {
     // Don't compare bits beyond length_.
-    uword mask =
-        static_cast<uword>(-1) >> (kBitsPerWord - (length_ % kBitsPerWord));
+    const intptr_t shift_size = (kBitsPerWord - length_) & (kBitsPerWord - 1);
+    const uword mask = static_cast<uword>(-1) >> shift_size;
     if ((data_[i] & mask) != (other.data_[i] & mask)) return false;
   }
   return true;