[LoopVectorize] Add tests for dereferenceable loads in more loops (llvm#118470)

* 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!

Author: david-arm