Replace _mm_loadu_si64 with _mm_loadl_epi64

Fix a compilation error with GCC 8.3.0. _mm_loadu_si64 was added to GCC
in GCC 9.0: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78782

Change-Id: I19b7220526ef85bb4450ffaf3dc78573962d9db4
diff --git a/aom_dsp/flow_estimation/x86/disflow_sse4.c b/aom_dsp/flow_estimation/x86/disflow_sse4.c
index 0a5680b..6ef40da 100644
--- a/aom_dsp/flow_estimation/x86/disflow_sse4.c
+++ b/aom_dsp/flow_estimation/x86/disflow_sse4.c
@@ -227,7 +227,7 @@
 
     __m128i warped = _mm_packs_epi32(sum0_rounded, sum1_rounded);
     __m128i src_pixels_u8 =
-        _mm_loadu_si64((__m128i *)&src[(y + i) * stride + x]);
+        _mm_loadl_epi64((__m128i *)&src[(y + i) * stride + x]);
     __m128i src_pixels = _mm_slli_epi16(_mm_cvtepu8_epi16(src_pixels_u8), 3);
 
     // Calculate delta from the target patch