Recommit "[VPlan] Insert Trunc/Exts for reductions directly in VPlan."

This reverts commit e4ea0997.

The recommit fixes a reported crash by adding a missing check to make
sure the cast recipes are only introduced when vectorizing.

Test coverage added in 3cac608f.
Original commit message:
   Update the code to create Trunc/Ext recipes directly in
    adjustRecipesForReductions instead of fixing it up later in
    fixReductions.

    This explicitly models the required conversions and also makes sure they
    are generated at the right place (instead of after the exit condition),
    hence the changes in a few tests.