[LV] Refactor cross-iteration phi's back-patching; NFC

This patch refactors the PHisToFix loop as follows:

- The loop itself now resides in its own method.
- The new method iterates on scalar-loop's header; the PHIsToFix map formerly
  propagated as an output parameter and filled during phi widening is removed.
- The code handling reductions is moved into its own method, similar to the
  existing fixFirstOrderRecurrence().

Differential Revision: https://reviews.llvm.org/D30755

llvm-svn: 297740