cmd/compile: don't walk field-name syntax in esc.go

Walking the field name as if it were an expression
caused a called to haspointers with a TFIELD, which panics.
Trigger was a field at a large offset within a large struct,
combined with a struct literal expression mentioning that
field.

Fixes #14405

Change-Id: I4589badae27cf3d7cf365f3a66c13447512f41f9
Reviewed-on: https://go-review.googlesource.com/19699
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue14405.go

@@ -0,0 +1,17 @@
+// compile
+
+// Copyright 2016 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Mention of field with large offset in struct literal causes crash
+package p
+
+type T struct {
+	Slice [1 << 20][]int
+	Ptr   *int
+}
+
+func New(p *int) *T {
+	return &T{Ptr: p}
+}