cmd/compile: make a better bogus line for empty infinite loops

The old recipe for making an infinite loop not be infinite
in the debugger could create an instruction (Prog) with a
line number not tied to any file (index == 0).  This caused
downstream failures in DWARF processing.

So don't do that.  Also adds a test, also adds a check+panic
to ensure that the next time this happens the error is less
mystifying.

Fixes #35652

Change-Id: I04f30bc94fdc4aef20dd9130561303ff84fd945e
Reviewed-on: https://go-review.googlesource.com/c/go/+/207613
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/issue35652.go

@@ -0,0 +1,15 @@
+// compile
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+func f() {
+	for true {
+		if true {
+			continue
+		}
+	}
+}