Remove all uses of Node.Val outside of the gc package.

A subsequent, automated commit in the Go 1.6 cycle
will unexport Node.Val.

No functional changes. Passes toolstash -cmp.

Change-Id: Ia92ae6a7766c83ab3e45c69edab24a9581c824f9
Reviewed-on: https://go-review.googlesource.com/9267Reviewed-by: Russ Cox <rsc@golang.org>