cmd/compile/internal/types2: avoid unpacking single-value LHS

cmd/compile/internal/types2: avoid unpacking single-value LHS

For ++/-- statements, we know that syntax.AssignStmt.Lhs is a
single expression. Avoid unpacking (and allocating a slice) in
that case. Minor optimization.

Change-Id: I6615fd12277b1cd7d4f8b86e0b9d39f27708c13e
Reviewed-on: https://go-review.googlesource.com/c/go/+/477915
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>