We can rematerialize only ops that have SP or SB as their only argument.
There are some ADDQconst(SP) that can be rematerialized, but are spilled/filled instead,
so mark addconst as rematerializeable. This shaves ~1kb from go tool.

Change-Id: Ib4cf4fe5f2ec9d3d7e5f0f77f1193eba66ca2f08
Reviewed-on: https://go-review.googlesource.com/54393
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>