[LoopPeel] Peel if it turns invariant loads dereferenceable.

This patch adds a new cost heuristic that allows peeling a single
iteration off read-only loops, if the loop contains a load that

    1. is feeding an exit condition,
    2. dominates the latch,
    3. is not already known to be dereferenceable,
    4. and has a loop invariant address.

If all non-latch exits are terminated with unreachable, such loads
in the loop are guaranteed to be dereferenceable after peeling,
enabling hoisting/CSE'ing them.

This enables vectorization of loops with certain runtime-checks, like
multiple calls to `std::vector::at` if the vector is passed as pointer.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D108114