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