cmd/compile/internal/typecheck: use types.IsMethodApplicable

cmd/compile/internal/typecheck: use types.IsMethodApplicable

In checking whether a type implements an interface, there's this
complex predicate spanning multiple lines, which is very obtuse.

So let's just use the helper function we already have in package types
instead.

Change-Id: I80f69d41c2bee8d6807601cf913840fa4f042b5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/521435
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>