cmd/compile: enforce that all nodes are used when generating DWARF

No particular need for this,
but it's nice to enforce invariants
when they are available.

Change-Id: Ia6fa88dc4116f65dac2879509746e123e2c1862a
Reviewed-on: https://go-review.googlesource.com/39201
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/pgen.go

@@ -327,7 +327,7 @@ func debuginfo(fnsym *obj.LSym, curfn interface{}) []*dwarf.Var {
 		switch n.Class {
 		case PAUTO:
 			if !n.Used() {
-				continue
+				Fatalf("debuginfo unused node (AllocFrame should truncate fn.Func.Dcl)")
 			}
 			name = obj.NAME_AUTO