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

The changes between (equivalent, and reviewed) go/types/operand.go
and operand.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

The primary differences compared to go/types are:
- use of syntax rather than go/ast package
- explicit mode for untyped nil (rather than relying on the type)

Change-Id: I0c9c1c6153c55cb0550096bd966c9f0f1c766734
Reviewed-on: https://go-review.googlesource.com/c/go/+/305571


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