[LoopVectorize] Add tests for dereferenceable loads in more loops

* Adds tests for strided accesses.
* Adds tests for reverse loops.

As part of this I've moved one of the negative tests from
load-deref-pred-align.ll into a new file
(load-deref-pred-neg-off.ll) because the pointer type had a
size of 16 bits and I realised it's probably not sensible for
allocas that are >16 bits in size!

I've also tweaked
scalar-steps-with-users-demanding-all-lanes-and-first-lane-only.ll
to use an alloca for the array access, rather than a global since
this more reliably leads to generation of dereferenceable loads.

Author: david-arm