Fix silk_VQ_WMat_EC_sse4_1().

During review of c95c9a04, I replaced a call to
 _mm_cvtepi8_epi32() with the OP_CVTEPI16_EPI32_M64() macro (note
 the 16 instead of 8).
Make a separate OP_CVTEPI8_EPI32_M32() macro and use that instead.

Thaks to Wei Zhou for the report.