We already combine const stores up-to MOVQstoreconst.
Combine 2 64-bit stores of const zero into 1 sse store of 128-bit zero.

Shaves significant (>1%) amount of code from go tool:
/localdisk/itocar/golang/bin/go 10334877
go_old 10388125 [53248 bytes]

global text (code) = 51041 bytes (1.343944%)
read-only data = 663 bytes (0.039617%)
Total difference 51704 bytes (0.873981%)

Change-Id: I7bc40968023c3a69f379b10fbb433cdb11364f1b
Reviewed-on: https://go-review.googlesource.com/56250
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Keith Randall <khr@golang.org>