[X86] Don't use a pmaddwd for vXi32 multiply if the inputs are zero extends...

[X86] Don't use a pmaddwd for vXi32 multiply if the inputs are zero extends from i8 or smaller without SSE4.1. Prefer to shrink the mul instead.

The zero extend will require two stages of unpacks to implement. So its better to shrink the multiply using pmullw and then extend that result back to v4i32 using a single unpack.

llvm-svn: 347149