cmd/compile/internal/types2: review of sanitize.go

Remove the "// UNREVIEWED" marker and add guards (as in go/types)
to prevent data races. To see the added guards, see compare patch
sets 3 and 4. The equivalent changes for go/types were done in
https://golang.org/cl/294411.

Change-Id: Ibef07eaae400bd32bff32b102cc743580290d135
Reviewed-on: https://go-review.googlesource.com/c/go/+/294510


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>