[VPlan] Use VPUser for VPWidenSelectRecipe operands (NFC).

VPWidenSelectRecipe already contains a VPUser, but it is not used. This
patch updates the code related to VPWidenSelectRecipe to use VPUser for
its operands.

Reviewers: Ayal, gilr, rengolin

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D80219