[LoopVectorizer] Prune VFs based on plan register pressure
Based on fhahn's work at https://github.com/llvm/llvm-project/pull/126437 .
This PR moves the register usage checking to after the plans are
created, so that any recipes that optimise register usage (such as
partial reductions) can be properly costed and not have their VF pruned
unnecessarily.
It involves changing some tests, notably removing one from
mve-known-tripcount.ll due to it not being vectorisable thanks to high
register pressure. tail-folding-reduces-vf.ll was modified to reduce its
register pressure but still test what was intended.
