[InstCombine] Add more complex folds for extractelement + stepvector

I have updated cheapToScalarize to also consider the case when
extracting lanes from a stepvector intrinsic. This required removing
the existing 'bool IsConstantExtractIndex' and passing in the actual
index as a Value instead. We do this because we need to know if the
index is <= known minimum number of elements returned by the stepvector
intrinsic. Effectively, when extracting lane X from a stepvector we
know the value returned is also X.

New tests added here:

  Transforms/InstCombine/vscale_extractelement.ll

Differential Revision: https://reviews.llvm.org/D106358