[VPlan] Replace BranchOnCount with BranchOnCond if TC <= UF * VF.

Try to simplify BranchOnCount to `BranchOnCond true` if TC <= UF * VF.

This is an alternative to D121899 which simplifies the VPlan directly
instead of doing so late in code-gen.

The potential benefit of doing this in VPlan is that this may help
cost-modeling in the future. The reason this is done in prepareToExecute
at the moment is that a single plan may be used for multiple VFs/UFs.

There are further simplifications that can be applied as follow ups:

1. Replace inductions with constants
2. Replace vector region with regular block.

Fixes #55354.

Depends on D126679.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D126680