cmd/compile: compute second method type at runtime

The type information for a method includes two variants: a func
without the receiver, and a func with the receiver as the first
parameter. The former is used as part of the dynamic interface
checks, but the latter is only returned as a type in the
reflect.Method struct.

Instead of computing it at compile time, construct it at run time
with reflect.FuncOf.

Using cl/20701 as a baseline,

	cmd/go: -480KB, (4.4%)
	jujud:  -5.6MB, (7.8%)

For #6853.

Change-Id: I1b8c73f3ab894735f53d00cb9c0b506d84d54e92
Reviewed-on: https://go-review.googlesource.com/20709
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/type.go

@@ -155,7 +155,6 @@ type method struct {
 	name    *string
 	pkgpath *string
 	mtyp    *_type
-	typ     *_type
 	ifn     unsafe.Pointer
 	tfn     unsafe.Pointer
 }