lavc/x86/hevc_add_res: Fix coeff overflow in ADD_RES_SSE_16_32_8

lavc/x86/hevc_add_res: Fix coeff overflow in ADD_RES_SSE_16_32_8

Fix overflow for coeff -32768 in function ADD_RES_SSE_16_32_8 with no
performance drop.(SSE2/AVX/AVX2)

./checkasm --test=hevc_add_res --bench

Mainline:
  - hevc_add_res.add_residual [OK]
    hevc_add_res_32x32_8_sse2: 127.5
    hevc_add_res_32x32_8_avx: 127.0
    hevc_add_res_32x32_8_avx2: 86.5

Add overflow test case:
  - hevc_add_res.add_residual [FAILED]

After:
  - hevc_add_res.add_residual [OK]
    hevc_add_res_32x32_8_sse2: 126.8
    hevc_add_res_32x32_8_avx: 128.3
    hevc_add_res_32x32_8_avx2: 86.8

Signed-off-by: Xu Guangxin <guangxin.xu@intel.com>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>