[X86] Use EVT::getVectorVT instead of changeVectorElementType in reduceVMULWidth.

Changing vector element type doesn't work for v6i32->v6i16 now
that v6i32 is an MVT and v6i16 is not.

I would like to fix this in changeVectorElementType, but you
need a LLVMContext to call getVectorVT which we can't get from
an MVT.

Fixes PR50709.