[X86] Custom type legalize v2i8/v2i16/v2i32 mul to use to pmuludq.

v2i8/v2i16/v2i32 are promoted to v2i64. pmuludq takes a v2i64 input and produces a v2i64 output. Since we don't about the upper bits of the type legalized multiply we can use the pmuludq to produce the multiply result for the bits we do care about.

llvm-svn: 346115