go.types, types2: factor out checking of LHS in variable assignment
Step towards disentangling assignment checking functionality.
In preparation for reverse inference of function type arguments,
but independently helpful in better separating concerns in the code.
Change-Id: I9bac9d8005090c00d9ae6c5cfa13765aacce6b12
Reviewed-on: https://go-review.googlesource.com/c/go/+/477855
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>