cmd/gc: fix computation of equality class of types.

cmd/gc: fix computation of equality class of types.

A struct with a single field was considered as equivalent to the
field type, which is incorrect is the field is blank.

Fields with padding could make the compiler think some
types are comparable when they are not.

Fixes #5698.

R=rsc, golang-dev, daniel.morsing, bradfitz, gri, r
CC=golang-dev
https://golang.org/cl/10271046