Don't fold (select C, (gep Ptr, Idx), Ptr) if C is vector but Idx is scalar

The folding rule (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C,
Idx, 0)) creates a malformed SELECT IR if C is a vector while Idx is scalar.

  SELECT VecC, ScalarIdx, 0

We could splat Idx to a vector but it defeats the purpose of
optimisation. Don't apply the folding rule in this case.

This fixes a regression from commit d561b6fb.