[SVE][LoopVectorize] Add option to disable tail-folding for reverse loops

If we use tail-folding for reverse loops that contain loads
and stores then we will need to reverse the loop predicate.
This patch adds a new 'reverse' sve-tail-folding option and
ensures they are not considered 'simple'.

I did this by adding a function called
containsDecreasingPointers to AArch64TargetTransformInfo.cpp
that searches all instructions in the loop for loads or
stores with negative strides.

Differential Revision: https://reviews.llvm.org/D146128