compiler: better error for x, x := 1, 2

Was
    assign.go:59:28: error: multiple assignments to x
Now
    assign.go:59:28: error: ‘x’ repeated on left side of :=

Change-Id: I4b69a96950a6077145822ef625fb2117424897f3
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273546
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed