go/types: fix potential bugs in santitizer pass
go/types: fix potential bugs in santitizer pass

This is a port of CL 317329 to go/types.

Change-Id: I1ba65284c91044f0ceed536da4149ef25e1f9502
Reviewed-on: https://go-review.googlesource.com/c/go/+/317291
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,golang/go