compiler: don't permit P.M if P is a pointer type
compiler: don't permit P.M if P is a pointer type

When lowering method expressions of the form "P.M" where
P is a pointer type (e.g. "type P *T") make sure we examine
the method set of P and not T during method lookup.

Fixes golang/go#15722.

Change-Id: Ic100a6a9d04efdc592d6acdb6734d3a906d8eeb2
Reviewed-on: https://go-review.googlesource.com/24843
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend