[DAGCombine] visitEXTRACT_VECTOR_ELT - add SimplifyDemandedBits multi use support

Similar to what we already do with SimplifyDemandedVectorElts, call SimplifyDemandedBits across all the extracted elements of the source vector, treating it as single use.

There's a minor regression in store-weird-sizes.ll which will be addressed in an upcoming SimplifyDemandedBits patch.