cmd/compile/internal/types2: better errors for invalid short var decls

- rewrite Checker.shortVarDecl core loop for clarity
- match compiler error messages (#43087)
- don't allow multiple identical redeclarations (#45652)

For #43087.
For #45652.

Change-Id: I8c3329a553aa104d7853fbaea8b88049bc9b3b88
Reviewed-on: https://go-review.googlesource.com/c/go/+/312170


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