go/types: fix recvPtr helper (follow-up on https://golang.org/cl/139422)
go/types: fix recvPtr helper (follow-up on https://golang.org/cl/139422)

The prior CL prepared go/types for the situation where methods might
not have a type-checked signature when being looked up. The respective
adjustments to recvPtr were not correct (but because so far method
signatures are type-checked in time, the bug didn't manifest itself).

Updates #23203.
Updates #26854.

Change-Id: I796691d11e6aac84396bdef802ad30715755fcc6
Reviewed-on: https://go-review.googlesource.com/c/139721
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

CAFxX/go,josharian/go,golang/go,akutz/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,akutz/go,cloudflare/go,josharian/go,akutz/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,akutz/go,cloudflare/go,golang/go,josharian/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,josharian/go,golang/go,CAFxX/go,nawawi/go,josharian/go,nawawi/go,josharian/go,akutz/go,akutz/go,golang/go,golang/go,golang/go,nawawi/go