cmd/compile: rework how fieldtrack is implemented

Shrinks gc.Type and gc.Func slightly.

Passes "GOEXPERIMENT=fieldtrack ./all.bash" and "go test -a
-toolexec='toolstash -cmp' -ldflags=-k=rsc.io/tmp/fieldtrack.tracked
rsc.io/tmp/fieldtrack".

Change-Id: I785fe8a18eb830d9867d34247e4cd41a6a7921d4
Reviewed-on: https://go-review.googlesource.com/20557
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -156,7 +156,7 @@ type Func struct {
 	Inldcl     Nodes   // copy of dcl for use in inlining
 	Closgen    int
 	Outerfunc  *Node
-	Fieldtrack []*Type
+	FieldTrack map[*Sym]struct{}
 	Outer      *Node // outer func for closure
 	Ntype      *Node // signature
 	Top        int   // top context (Ecall, Eproc, etc)