[LoopVectorize] Make VPWidenCanonicalIVRecipe::execute work for scalable vectors

The code in VPWidenCanonicalIVRecipe::execute only worked for fixed-width
vectors due to the way we generate the values per lane. This patch changes
the code to use a combination of vector splats and step vectors to get
the same result. This then works for both fixed-width and scalable vectors.

Tests that exercise this code path for scalable vectors have been added here:

  Transforms/LoopVectorize/AArch64/sve-tail-folding.ll

Differential Revision: https://reviews.llvm.org/D113180