cmd/compile: fix -N build

The decomposer of builtin types is confused by having structs
still around from the user-type decomposer.  They're all dead though,
so just enabling a deadcode pass fixes things.

Change-Id: I2df6bc7e829be03eabfd24c8dda1bff96f3d7091
Reviewed-on: https://go-review.googlesource.com/21839
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -230,9 +230,9 @@ var passes = [...]pass{
 	{name: "early deadcode", fn: deadcode}, // remove generated dead code to avoid doing pointless work during opt
 	{name: "short circuit", fn: shortcircuit},
 	{name: "decompose user", fn: decomposeUser, required: true},
-	{name: "opt", fn: opt, required: true},           // TODO: split required rules and optimizing rules
-	{name: "zero arg cse", fn: zcse, required: true}, // required to merge OpSB values
-	{name: "opt deadcode", fn: deadcode},             // remove any blocks orphaned during opt
+	{name: "opt", fn: opt, required: true},               // TODO: split required rules and optimizing rules
+	{name: "zero arg cse", fn: zcse, required: true},     // required to merge OpSB values
+	{name: "opt deadcode", fn: deadcode, required: true}, // remove any blocks orphaned during opt
 	{name: "generic cse", fn: cse},
 	{name: "phiopt", fn: phiopt},
 	{name: "nilcheckelim", fn: nilcheckelim},

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

@@ -79,7 +79,7 @@ func decomposeBuiltIn(f *Func) {
 			}
 			delete(f.NamedValues, name)
 		case t.Size() > f.Config.IntSize:
-			f.Unimplementedf("undecomposed named type %s", t)
+			f.Unimplementedf("undecomposed named type %s %s", name, t)
 		default:
 			newNames = append(newNames, name)
 		}