[llvm] PreISelIntrinsicLowering: Lower llvm.exp to a loop if scalable vec arg (PR #117568)

[Stephen Long via llvm-commits]

steplong wrote:

@arsenm I added the fixed vector test case, but if we decide to only do this for scalable vectors, then I'll remove it. Let me know how we should do the legality check as I'm new to this.

https://github.com/llvm/llvm-project/pull/117568