cmd/asm: uncomment tests for amd64 PHADD{SW,W}, PHSUB{D,SW,W}

cmd/asm: uncomment tests for amd64 PHADD{SW,W}, PHSUB{D,SW,W}

Instructions added in https://golang.org/cl/18853

2nd change out of 3 to cover AMD64 SSSE3 instruction set in Go asm.
This commit does not actually add any new instructions, only
enables some test cases.

Change-Id: I9596435b31ee4c19460a51dd6cea4530aac9d198
Reviewed-on: https://go-review.googlesource.com/56835
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>