[SLP][NFCI] Address issues seen in downstream Coverity scan. (#93757)

- Prevent null dereference: if the Mask given to
  `ShuffleInstructionBuilder::adjustExtracts()` is empty or all-poison,
  then `VecBase` will be `nullptr` and the call to
  `castToScalarTyElem(VecBase)` will dereference it. Add an assert
  to guard against this.

- Prevent use of uninitialized scalar: in the unlikely event that
  `CandidateVFs` is empty, then `AnyProfitableGraph` will be
  uninitialized in `if` condition following the loop. (This seems like a
  false-positive, but I submitted this change anyways as initializing
  bools costs nothing and is generally good practice)