cmd/compile: fix interaction between GOEXPERIMENT=fieldtrack and race detector

Tested by hand.
Only lines of code changing are protected by Fieldtrack_enabled > 0,
which is never true in standard Go distributions.

Fixes #12171.

Change-Id: I963b9997dac10829db8ad4bfc97a7d6bf14b55c6
Reviewed-on: https://go-review.googlesource.com/13676Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -2532,7 +2532,7 @@ func lookdot(n *Node, t *Type, dostrcmp int) *Type {
 		n.Xoffset = f1.Width
 		n.Type = f1.Type
 		if obj.Fieldtrack_enabled > 0 {
-			dotField[typeSym{t, s}] = f1
+			dotField[typeSym{t.Orig, s}] = f1
 		}
 		if t.Etype == TINTER {
 			if Isptr[n.Left.Type.Etype] {

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

@@ -3873,7 +3873,7 @@ func usefield(n *Node) {
 	if Isptr[t.Etype] {
 		t = t.Type
 	}
-	field := dotField[typeSym{t, n.Right.Sym}]
+	field := dotField[typeSym{t.Orig, n.Right.Sym}]
 	if field == nil {
 		Fatal("usefield %v %v without paramfld", n.Left.Type, n.Right.Sym)
 	}