[dev.ssa] cmd/compile/ssa: add checks for nil args in values

These additional checks were useful in
tracking down the broken build (CL 11238).

This CL does not fix the build, sadly.

Change-Id: I34de3bed223f450aaa97c1cadaba2e4e5850050b
Reviewed-on: https://go-review.googlesource.com/11681
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -599,7 +599,11 @@ func (s *state) addr(n *Node) *ssa.Value {
 			return s.entryNewValue1A(ssa.OpAddr, Ptrto(n.Type), aux, s.sb)
 		case PPARAM, PPARAMOUT, PAUTO:
 			// parameter/result slot or local variable
-			return s.decladdrs[n]
+			v := s.decladdrs[n]
+			if v == nil {
+				s.Fatalf("addr of undeclared ONAME %v. declared: %v", n, s.decladdrs)
+			}
+			return v
 		case PAUTO | PHEAP:
 			return s.expr(n.Name.Heapaddr)
 		default:

src/cmd/compile/internal/ssa/check.go

@@ -92,6 +92,12 @@ func checkFunc(f *Func) {
 		}
 
 		for _, v := range b.Values {
+			for _, arg := range v.Args {
+				if arg == nil {
+					f.Fatalf("value %v has nil arg", v.LongString())
+				}
+			}
+
 			if valueMark[v.ID] {
 				f.Fatalf("value %s appears twice!", v.LongString())
 			}

src/cmd/compile/internal/ssa/print.go

@@ -49,7 +49,9 @@ func fprintFunc(w io.Writer, f *Func) {
 					continue
 				}
 				for _, w := range v.Args {
-					if w.Block == b && !printed[w.ID] {
+					// w == nil shouldn't happen, but if it does,
+					// don't panic; we'll get a better diagnosis later.
+					if w != nil && w.Block == b && !printed[w.ID] {
 						continue outer
 					}
 				}