[NFC][LoopVectorize] Change getStepVector to take a Value* for the StartIdx

This patch changes the definition of getStepVector from:

  Value *getStepVector(Value *Val, int StartIdx, Value *Step, ...

to

  Value *getStepVector(Value *Val, Value *StartIdx, Value *Step, ...

because:

1. it seems inconsistent to pass some values as Value* and some as
   integer, and
2. future work will require the StartIdx to be an expression made up
   of runtime calculations of the VF.

In widenIntOrFpInduction I've changed the code to pass in the
value returned from getRuntimeVF, but the presence of the assert:

  assert(!VF.isScalable() && "scalable vectors not yet supported.");

means that currently this code path is only exercised for fixed-width
VFs and so the patch is still NFC.

Differential revision: https://reviews.llvm.org/D111882