[TLI] SimplifyDemandedVectorElts(): handle SCALAR_TO_VECTOR(EXTRACT_VECTOR_ELT(?, 0))

Iff we have `SCALAR_TO_VECTOR` (and we demand it's only defined 0'th element),
and said scalar was produced by `EXTRACT_VECTOR_ELT` from the 0'th element
of some vector, then we can just continue traversal into said source vector.

This comes up in X86 vector uniform shift lowering.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D104250