[LAA] Always use DepCands when grouping runtime checks. (#91196)

Update groupChecks to always use DepCands to try and merge runtime
checks. DepCands contains the dependency partition, grouping together
all accessed pointers to he same underlying objects.

If we computed the dependencies, We only need to check accesses to the
same underlying object, if there is an unknown dependency for this
underlying object; otherwise we already proved that all accesses withing
the underlying object are safe w.r.t. vectorization and we only need to
check that accesses to the underlying object don't overlap with accesses
to other underlying objects.

To ensure runtime checks are generated for the case with unknown
dependencies, remove equivalence classes containing accesses involved in
unknown dependencies.

This reduces the number of runtime checks needed in case non-constant
dependence distances are found, and is in preparation for removing the
restriction that the accesses need to have the same stride which was
added in https://github.com/llvm/llvm-project/pull/88039.

PR: https://github.com/llvm/llvm-project/pull/91196