reuse cc->call

I noticed that in case of cache misshit, re-calculated cc->me can
be the same method entry than the pevious one.  That is an okay
situation but can't we partially reuse the cache, because cc->call
should still be valid then?

One thing that has to be special-cased is when the method entry
gets amended by some refinements.  That happens behind-the-scene
of call cache mechanism.  We have to check if cc->me->def points to
the previously saved one.

Calculating -------------------------------------
                          trunk        ours
vm2_poly_same_method     1.534M      2.025M i/s -      6.000M times in 3.910203s 2.962752s

Comparison:
             vm2_poly_same_method
                ours:   2025143.9 i/s
               trunk:   1534447.2 i/s - 1.32x  slower

1 files changed, 25 insertions, 0 deletions

diff --git a/benchmark/vm2_poly_same_method.yml b/benchmark/vm2_poly_same_method.yml
new file mode 100644
index 0000000000..867c433cf8
--- /dev/null
+++ b/benchmark/vm2_poly_same_method.yml
@@ -0,0 +1,25 @@
+prelude: |
+  module AR; end
+  class AR::Base
+    def create_or_update
+      nil
+    end
+    def save
+      create_or_update
+    end
+  end
+  class Foo < AR::Base; end
+  class Bar < AR::Base; end
+  o1 = Foo.new
+  o2 = Bar.new
+benchmark:
+  vm2_poly_same_method: |
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+    o1.save; o2.save;
+loop_count: 6000000