[VPlan] Replace IR based truncateToMinimalBitwidths with VPlan version.

This patch replaces the IR based truncateToMinimalBitwidths with a VPlan
version. This has 3 benefits:
1) the VPlan-based version is simpler; we don't need to implement
   special codegen for each supported instruction type like the IR based
   one.
2) Removes a dependency on the cost-model after VPlan execution and
3) Removes a use of getVPValue that uses underlying values after VPlan
   execution (See removed FIXME).

Depends on D149081.

Depends on D149079.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D149903