cmd/compile: only optimize chained Moves on disjoint stack mem

This optimization is not sound if A, B, or C
might overlap with each other.

Thanks to Michael Munday for pointing this
out during the review of CL 143479.

This reduces the number of times this optimization
triggers during make.bash from 386 to 74.

This is unfortunate, but I don't see an obvious way around it,
short of souping up the disjointness analysis.

name        old object-bytes  new object-bytes  delta
Template          507kB ± 0%        507kB ± 0%   +0.13%  (p=0.008 n=5+5)
Unicode           225kB ± 0%        225kB ± 0%     ~     (all equal)
GoTypes          1.85MB ± 0%       1.85MB ± 0%   +0.02%  (p=0.008 n=5+5)
Flate             328kB ± 0%        328kB ± 0%     ~     (all equal)
GoParser          402kB ± 0%        402kB ± 0%     ~     (all equal)
Reflect          1.41MB ± 0%       1.41MB ± 0%     ~     (all equal)
Tar               457kB ± 0%        458kB ± 0%   +0.20%  (p=0.008 n=5+5)
XML               600kB ± 0%        601kB ± 0%   +0.03%  (p=0.008 n=5+5)

Change-Id: Ida408cb627145ba9faf473a78606f050c2f3f51c
Reviewed-on: https://go-review.googlesource.com/c/145208


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>