[SVE][Analysis] Tune the cost model according to the tune-cpu attribute

This patch introduces a new function:

  AArch64Subtarget::getVScaleForTuning

that returns a value for vscale that can be used for tuning the cost
model when using scalable vectors. The VScaleForTuning option in
AArch64Subtarget is initialised according to the following rules:

1. If the user has specified the CPU to tune for we use that, else
2. If the target CPU was specified we use that, else
3. The tuning is set to "generic".

For CPUs of type "generic" I have assumed that vscale=2.

New tests added here:

  Analysis/CostModel/AArch64/sve-gather.ll
  Analysis/CostModel/AArch64/sve-scatter.ll
  Transforms/LoopVectorize/AArch64/sve-strict-fadd-cost.ll

Differential Revision: https://reviews.llvm.org/D110259