[VPlan] Add plain text (not DOT's digraph) dumps
I foresee two uses for this:
- It's easier to use those in debugger.
- Once we start implementing more VPlan-to-VPlan transformations (especially inner loop massaging stuff), using the vectorized LLVM IR as CHECK targets in LIT test would become too obscure. I can imagine that we'd want to CHECK against VPlan dumps after multiple transformations instead. That would be easier with plain text dumps than with DOT format.
Reviewed By: fhahn
Differential Revision: https://reviews.llvm.org/D96628