Improve forward-looking desired register calculations.
It is now inter-block and handles a bunch more cases.

Fixes #14504
Fixes #14828
Fixes #15254

Change-Id: Ic240fa0ec6a779d80f577f55c8a6c4ac8c1a940a
Reviewed-on: https://go-review.googlesource.com/22160
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>