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

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

The primary differences to go/types/assignments.go are:
- use of syntax instead of go/ast package
- no reporting of error codes (for now)
- different handling of nil values (we can't use Typ[UntypedNil]
  to represent an untyped nil because types2 gives such nil values
  context-dependent types)

Change-Id: I5d8a58f43ca8ed2daa060c46842a6ebc11b3cb35
Reviewed-on: https://go-review.googlesource.com/c/go/+/304051
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/assignments.go

@@ -1,4 +1,3 @@
-// UNREVIEWED
 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
@@ -131,6 +130,8 @@ func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
+		// Note: This was reverted in go/types (https://golang.org/cl/292751).
+		// TODO(gri): decide what to do (also affects test/run.go exclusion list)
 		lhs.used = true // avoid follow-on "declared but not used" errors
 		return nil
 	}