VP9_LPF_VERTICAL_16_DUAL_SSE2 optimization

VP9_LPF_VERTICAL_16_DUAL_SSE2 optimization

The vp9_lpf_vertical_16_dual function optimized for x86 32bit target. The hot code in that function was caused by the call to the transpose8x16.
The gcc generated assembly created uneeded fills and spills to the stack. By interleaving 2 loads and unpack instructions, in addition to hoisting the consumer
instruction closer to the producer instructions, we eliminated most of the fills and spills and improve the function-level performance by 17%.
credit for writing the function as well as finding the root cause goes to Erik Niemeyer (erik.a.niemeyer@intel.com)

Change-Id: I6173cf53956d52918a047d1c53d9a673f952ec46