reflect: fix FieldByNameFunc

The existing algorithm did not properly propagate the type
count from one level to the next, and as a consequence it
missed collisions.

Properly propagate multiplicity (count) information to the
next level.

benchmark                old ns/op    new ns/op    delta
BenchmarkFieldByName1          182          180   -1.10%
BenchmarkFieldByName2         6273         6183   -1.43%
BenchmarkFieldByName3        49267        46784   -5.04%

Fixes #4355.

R=rsc
CC=golang-dev
https://golang.org/cl/6821094

src/pkg/reflect/all_test.go

@@ -1694,6 +1694,20 @@ type S13 struct {
 	S8
 }
 
+// The X in S15.S11.S1 and S16.S11.S1 annihilate.
+type S14 struct {
+	S15
+	S16
+}
+
+type S15 struct {
+	S11
+}
+
+type S16 struct {
+	S11
+}
+
 var fieldTests = []FTest{
 	{struct{}{}, "", nil, 0},
 	{struct{}{}, "Foo", nil, 0},
@@ -1719,6 +1733,7 @@ var fieldTests = []FTest{
 	{S5{}, "Y", []int{2, 0, 1}, 0},
 	{S10{}, "X", nil, 0},
 	{S10{}, "Y", []int{2, 0, 0, 1}, 0},
+	{S14{}, "X", nil, 0},
 }
 
 func TestFieldByIndex(t *testing.T) {