[llvm] [AArch64] Add flag to control unrolling for small multi-exit loops (PR #131998)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

If the intention is to enable this for all AArch64 targets then yes I agree this means we only need to be able to disable the option. To avoid confusion I will clarify my "we don't want a target feature" comment.  Here I'm referring to public flags and specifically mean we don't want something at the -march level. `-target-feature` is effectively a secret flag and so I'm not against that.

https://github.com/llvm/llvm-project/pull/131998