cmd/compile: Fix `internal compiler: getinarg: not a func` when returning invalid...

cmd/compile: Fix `internal compiler: getinarg: not a func` when returning invalid interface.

Internal error arose from calling methodfunc on a invalid interface
field during the implements check. int obviously isn't a function,
and errors on getinarg...

for im := iface.Type; im != nil; im = im.Down {
  imtype = methodfunc(im.Type, nil)
  // ...
}

Fix handles the internal compiler error, but does not throw an
additional error, i.e. the following code will error on the I
interface, but type A will pass the implements check since
'Read(string) string' is implemented and 'int' is skipped

type I interface {
  Read(string) string
  int
}

type A struct {
}

func (a *A) Read(s string) string {
  return s
}

func New() I {
  return new(A)
}

Fixes #10975

Change-Id: I4b54013afb2814db3f315515f0c742d8631ca500
Reviewed-on: https://go-review.googlesource.com/13747
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>