[AArch64][LoopVectorize] Use upper bound trip count instead of the constant TC...

[AArch64][LoopVectorize] Use upper bound trip count instead of the constant TC when choosing max VF (#67697)

This patch is based off of
https://github.com/llvm/llvm-project/pull/67543.

We are currently using the exact trip count to make decisions regarding
the maximum VF. We can instead use the upper bound TC, which will be the
same as the constant trip count when that is known.