go/types, types2: mechanically replace read accesses to operand.typ_

go/types, types2: mechanically replace read accesses to operand.typ_

This provides a hook to enforce that operand.typ is only observed where
operand.mode is not invalid.

For #76110

Change-Id: I915f3ac09dc10bfe3f9f688d6190ad58e195ddcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/741220
Auto-Submit: Mark Freeman <markfreeman@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>