cmd/compile: fix fieldtrack experiment

The ODOTPTRs introduced in CL #19814 don't have field names,
just offsets.  The fieldtrack experiment crashes when
examining them.  Instead, just ignore them.  We'll never track
these fields anyway.

It would be nice to have the runtime type struct build in the
compiler (like we do sudog, for example) so we could use its
fieldnames.  Doesn't seem worth it just for this CL.

Change-Id: I5e75024f5a8333eb7439543b3f466ea40213a1b9
Reviewed-on: https://go-review.googlesource.com/20157Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -3763,6 +3763,11 @@ func usefield(n *Node) {
 	case ODOT, ODOTPTR:
 		break
 	}
+	if n.Right == nil {
+		// No field name.  This DOTPTR was built by the compiler for access
+		// to runtime data structures.  Ignore.
+		return
+	}
 
 	t := n.Left.Type
 	if Isptr[t.Etype] {