i386: Expand mul<VI8_AVX2> earlier

i386: Expand mul<VI8_AVX2> earlier

Move the expansion code to i386.c next to mulv4si3.  Eliminate
one shift by adding the highparts before shifting.  Correct costs.

        * config/i386/sse.md (mul<VI8_AVX2>3): Change from insn_and_split
        to expander; move guts to ...
        * config/i386/i386.c (ix86_expand_sse2_mulvxdi3): ... here.  Add
        highparts before shifting up.
        * config/i386/i386-protos.h: Update.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@189005 138bc75d-0d04-0410-961f-82ee72b054a4