cmd/compile: remove useless block-indirection in type switch

Previously, when emitting type switches without an explicit "case nil"
clause, we would emit:

    if x == nil { goto Lnil }
    ...
    Lnil: goto Ldefault

But we can instead just emit:

    if x == nil { goto Ldefault }

Doesn't pass toolstash-check; seems like it causes some harmless
instruction scheduling changes.

Change-Id: Ie233dda26756911e93a08b3db40407ba38694c62
Reviewed-on: https://go-review.googlesource.com/c/go/+/199644
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -588,20 +588,10 @@ func walkTypeSwitch(sw *Node) {
 	if defaultGoto == nil {
 		defaultGoto = br
 	}
-
-	if nilGoto != nil {
-		ifNil.Nbody.Set1(nilGoto)
-	} else {
-		// TODO(mdempsky): Just use defaultGoto directly.
-
-		// Jump to default case.
-		label := autolabel(".s")
-		ifNil.Nbody.Set1(nodSym(OGOTO, nil, label))
-		// Wrap default case with label.
-		blk := nod(OBLOCK, nil, nil)
-		blk.List.Set2(nodSym(OLABEL, nil, label), defaultGoto)
-		defaultGoto = blk
+	if nilGoto == nil {
+		nilGoto = defaultGoto
 	}
+	ifNil.Nbody.Set1(nilGoto)
 
 	s.Emit(&sw.Nbody)
 	sw.Nbody.Append(defaultGoto)