[VPlan] Clean up tryToCreate(Widen)Recipe. (NFC)

This patch includes some clean-ups to tryToCreateRecipe, suggested in
D77973.

It includes:
  * Renaming tryToCreateRecipe to tryToCreateWidenRecipe.
  * Move VPBB insertion logic to caller of tryToCreateWidenRecipe.
  * Hoists instruction checks to tryToCreateWidenRecipe, making it
    clearer which instructions are handled by which recipe, simplifying
    the checks by using early exits.
  * Split up handling of induction PHIs and truncates using inductions.

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D78287