Cherry pick vp8 halfpix variance fix

source/libvpx/vpx_dsp/vpx_dsp.mk

 ##
 ## Copyright (c) 2015 The WebM project authors. All Rights Reserved.
 ##
 ##  Use of this source code is governed by a BSD-style license
 ##  that can be found in the LICENSE file in the root of the source
 ##  tree. An additional intellectual property rights grant can be found
 ##  in the file PATENTS.  All contributing project authors may
 ##  be found in the AUTHORS file in the root of the source tree.
 ##
 
@@ skipping 261 matching lines @@
 DSP_SRCS-$(HAVE_MEDIA)  += arm/variance_media$(ASM)
 DSP_SRCS-$(HAVE_NEON)   += arm/subpel_variance_neon.c
 DSP_SRCS-$(HAVE_NEON)   += arm/variance_neon.c
 
 DSP_SRCS-$(HAVE_MSA)    += mips/variance_msa.c
 DSP_SRCS-$(HAVE_MSA)    += mips/sub_pixel_variance_msa.c
 
 DSP_SRCS-$(HAVE_MMX)    += x86/variance_mmx.c
 DSP_SRCS-$(HAVE_MMX)    += x86/variance_impl_mmx.asm
 DSP_SRCS-$(HAVE_SSE2)   += x86/variance_sse2.c  # Contains SSE2 and SSSE3
+DSP_SRCS-$(HAVE_SSE2)   += x86/halfpix_variance_sse2.c
+DSP_SRCS-$(HAVE_SSE2)   += x86/halfpix_variance_impl_sse2.asm
 DSP_SRCS-$(HAVE_AVX2)   += x86/variance_avx2.c
 DSP_SRCS-$(HAVE_AVX2)   += x86/variance_impl_avx2.c
 
 ifeq ($(CONFIG_USE_X86INC),yes)
 DSP_SRCS-$(HAVE_SSE2)   += x86/subpel_variance_sse2.asm  # Contains SSE2 and SSSE3
 endif  # CONFIG_USE_X86INC
 
 ifeq ($(CONFIG_VP9_HIGHBITDEPTH),yes)
 DSP_SRCS-$(HAVE_SSE2)   += x86/highbd_variance_sse2.c
 DSP_SRCS-$(HAVE_SSE2)   += x86/highbd_variance_impl_sse2.asm
 ifeq ($(CONFIG_USE_X86INC),yes)
 DSP_SRCS-$(HAVE_SSE2)   += x86/highbd_subpel_variance_impl_sse2.asm
 endif  # CONFIG_USE_X86INC
 endif  # CONFIG_VP9_HIGHBITDEPTH
 endif  # CONFIG_ENCODERS || CONFIG_POSTPROC || CONFIG_VP9_POSTPROC
 
 DSP_SRCS-no += $(DSP_SRCS_REMOVE-yes)
 
 DSP_SRCS-yes += vpx_dsp_rtcd.c
 DSP_SRCS-yes += vpx_dsp_rtcd_defs.pl
 
 $(eval $(call rtcd_h_template,vpx_dsp_rtcd,vpx_dsp/vpx_dsp_rtcd_defs.pl))