[LoopVectorize][AArch64] Enable ordered reductions by default for AArch64

I have added a new TTI interface called enableOrderedReductions() that
controls whether or not ordered reductions should be enabled for a
given target. By default this returns false, whereas for AArch64 it
returns true and we rely upon the cost model to make sensible
vectorisation choices. It is still possible to override the new TTI
interface by setting the command line flag:

  -force-ordered-reductions=true|false

I have added a new RUN line to show that we use ordered reductions by
default for SVE and Neon:

  Transforms/LoopVectorize/AArch64/strict-fadd.ll
  Transforms/LoopVectorize/AArch64/scalable-strict-fadd.ll

Differential Revision: https://reviews.llvm.org/D106653