[dev.ssa] cmd/compile: flag recomputing: find original values correctly

[dev.ssa] cmd/compile: flag recomputing: find original values correctly

We "spill" flag values by recomputing them from their original
inputs.  The "find original inputs" part of the algorithm was
a hack.  It was broken by rematerialization.  This change does
the real job of keeping track of original values for each
spill/restore/flagrecompute/rematerialization we issue.

Change-Id: I95088326a4ee4958c98148b063e518c80e863e4c
Reviewed-on: https://go-review.googlesource.com/16500
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>