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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

> I was thinking that someone might be targeting a generic CPU with a particular architecture (perhaps this happens when targeting Android?), but still want a way to get the benefit of unrolling multi-exit loops such as std::find. Having said that, if the consensus is to avoid introducing a flag I can remove it?

Having the option sounds useful for debugging - we can use it to disable the unrolling if we find issues. Perhaps rename it to -aarch64-small-multi-exit-loop-unroll-factor?

I gave this a try and it seemed OK. The code size, which was the thing I was worried about, did not increase by too much.

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