[VPlan] Add field to track if intrinsic should be used for call. (NFC)

This patch moves the cost-based decision whether to use an intrinsic or
library call to the point where the recipe is created. This untangles
code-gen from the cost model and also avoids doing some extra work as
the information is already computed at construction.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D132585