Disable aom_sum_squares_2d_i16_sse2

aom_sum_squares_2d_i16_sse2 doesn't work correctly (e.g. aligned load
unaligned data). It is disabled for now, and will be re-enabled after
it is fixed.

Fix issue #749
diff --git a/aom_dsp/aom_dsp_rtcd_defs.pl b/aom_dsp/aom_dsp_rtcd_defs.pl
index 9e6b5b1..ca7b7a0 100755
--- a/aom_dsp/aom_dsp_rtcd_defs.pl
+++ b/aom_dsp/aom_dsp_rtcd_defs.pl
@@ -630,7 +630,7 @@
     # Sum of Squares
     #
     add_proto qw/uint64_t aom_sum_squares_2d_i16/, "const int16_t *src, int stride, int width, int height";
-    specialize qw/aom_sum_squares_2d_i16 sse2 avx2 neon/;
+    specialize qw/aom_sum_squares_2d_i16 avx2 neon/;
 
     add_proto qw/uint64_t aom_sum_squares_i16/, "const int16_t *src, uint32_t N";
     specialize qw/aom_sum_squares_i16 sse2/;
diff --git a/test/sum_squares_test.cc b/test/sum_squares_test.cc
index 30fac46..da274af 100644
--- a/test/sum_squares_test.cc
+++ b/test/sum_squares_test.cc
@@ -157,8 +157,9 @@
 
 TEST_P(SumSquaresTest, DISABLED_Speed) { RunSpeedTest(); }
 
-#if HAVE_SSE2
-
+#if 0  // HAVE_SSE2
+// As reported in issue 749 (https://gitlab.com/AOMediaCodec/avm/-/issues/749),
+// sse2 version needs to be fixed and then re-enabled.
 INSTANTIATE_TEST_SUITE_P(
     SSE2, SumSquaresTest,
     ::testing::Values(TestFuncs(&aom_sum_squares_2d_i16_c,