Fix error reported by valgrind
Fix error reported by valgrind

It seems valgrind has some problem to
support avx2 instruction _mm256_mulhrs_epi16,
which cause valgrind run out temporary storage.

Replace this instruction with two instructions
(add and shift right) can fix this error, at
the sacrifice of encoder speed.

BUG=aomedia:2015
BUG=aomedia:2016

Change-Id: Ibc027d35f2ee2299bc40deb7f0d88d825c2ebcc2

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom