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Use 'packssdw' for loading tran_low_t values

This matches bitdepth_conversion_sse2.asm and produces substantially
better assembly. The old way had lots of 'movzwl' and 'shl' and storing
back to memory before loading into an xmm register.

Change-Id: Ib33e35354dfd691a4f8b1e39f4dbcbb14cd5302b

--- a/vpx_dsp/x86/bitdepth_conversion_sse2.h

+++ b/vpx_dsp/x86/bitdepth_conversion_sse2.h

@@ -16,13 +16,12 @@

 #include "vpx/vpx_integer.h"

 #include "vpx_dsp/vpx_dsp_common.h"

-// Load 8 16 bit values. If the source is 32 bits then cast down.

-// This does not saturate values. It only truncates.

+// Load 8 16 bit values. If the source is 32 bits then pack down with

+// saturation.

 static INLINE __m128i load_tran_low(const tran_low_t *a) {

 #if CONFIG_VP9_HIGHBITDEPTH

-  return _mm_setr_epi16((int16_t)a[0], (int16_t)a[1], (int16_t)a[2],

-                        (int16_t)a[3], (int16_t)a[4], (int16_t)a[5],

-                        (int16_t)a[6], (int16_t)a[7]);

+  const __m128i a_low = _mm_load_si128((const __m128i *)a);

+  return _mm_packs_epi32(a_low, *(const __m128i *)(a + 4));

 #else

   return _mm_load_si128((const __m128i *)a);

 #endif