cmd/compile/internal/ssa: prefer non-indexed stores on amd64

cmd/compile/internal/ssa: prefer non-indexed stores on amd64

We sometimes generate code like this:

v473 = MOVQconst <uintptr> // constant..
v580 = MOVBstoreidx1 <mem> v1056 v473 v819 v491 // ..only used as an index

Rewrite indexed stores to non-indexed version, where possible.
This allows to eliminate  const->register move, reducing codesize and lowering register pressure.

Change-Id: Id5fed801dffe3f1a80876b8d8bd43775a7c942b2
Reviewed-on: https://go-review.googlesource.com/105295
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>