[DAG] SimplifyMultipleUseDemandedBits - drop unnecessary *_EXTEND_VECTOR_INREG cases

For little endian targets, if we only need the lowest element and none of the extended bits then we can just use the (bitcasted) source vector directly.

We already do this in SimplifyDemandedBits, this adds the SimplifyMultipleUseDemandedBits equivalent.