[X86] Add DAG combine to replace vXi64 vzext_movl+scalar_to_vector with vYi32...

[X86] Add DAG combine to replace vXi64 vzext_movl+scalar_to_vector with vYi32 vzext_movl+scalar_to_vector if the upper 32 bits of the scalar are zero.

We can just use a 32-bit copy and zero in the SSE domain when we
zero the upper bits.

Remove an isel pattern that becomes dead with this.