cmd/compile/internal/syntax: add test case for invalid label use
This case is not properly handled by the type checkers (see issue)
but the compiler uses the parser's label checking so it works as
expected.
For #70974.
Change-Id: I0849376bf7514a9a7730846649c3fe28c91f44ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/640895
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>