This patch replaces the IR based truncateToMinimalBitwidths with a VPlan
version. This has 2 benefits:
- the VPlan-based version is simpler; we don't need to implement special codegen for each supported instruction type like the IR based one.
- Removes a dependency on the cost-model after VPlan execution and
- Removes a use of getVPValue that uses underlying values after VPlan execution (See removed FIXME).
Depends on D149081.
Depends on D149079.
Retain a comment explaining why replicate recipes are not truncated?