cmd/compile: use better fatal message for staticValue1

cmd/compile: use better fatal message for staticValue1

So the position of the wrong assignment statement will be reported,
instead of using incorrect base.Pos one.

Notice while fixing issue #73823.

Change-Id: I53f240bf99d11b5f5082ee4ca0903d9f099881b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/675495
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>