cmd/compile: check for iteration after range func loop exit

When this happens, panic.

This is a revised version of a check that used #next,
where this one instead uses a per-loop #exit flag,
and catches more problematic iterators.

Updates #56413.
Updates #61405.

Change-Id: I6574f754e475bb67b9236b4f6c25979089f9b629
Reviewed-on: https://go-review.googlesource.com/c/go/+/540263
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/runtime/panic.go

@@ -296,6 +296,13 @@ func deferproc(fn func()) {
 	// been set and must not be clobbered.
 }
 
+var rangeExitError = error(errorString("range function continued iteration after exit"))
+
+//go:noinline
+func panicrangeexit() {
+	panic(rangeExitError)
+}
+
 // deferrangefunc is called by functions that are about to
 // execute a range-over-function loop in which the loop body
 // may execute a defer statement. That defer needs to add to