[VPlan] Keep induction recipes in header.

This patch updates recipe creation to ensure all
VPWidenIntOrFpInductionRecipes are in the header block. At the moment,
new induction recipes can be created in different blocks when trying to
optimize casts and induction variables.

Having all induction recipes in the header makes it easier to
analyze/transform them in VPlan.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D111300