[X86][AVX] lowerBuildVectorAsBroadcast - improve i64 BROADCASTM lowering on 32-bit targets

We already handle the the cases where we have a 'zero extended splat' build vector (a, 0, 0, 0, a, 0, 0, 0, ...) but were missing the case where the 'a' scalar was zero-extended as well - such as i64 -> vXi64 splat cases on 32-bit targets.