Use a more precise computation of next use.  It properly
detects lifetime holes and deallocates values during those holes.
It also uses a more precise version of distance to next use which
affects which values get spilled.

Change-Id: I49eb3ebe2d2cb64842ecdaa7fb4f3792f8afb90b
Reviewed-on: https://go-review.googlesource.com/16760
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>