[llvm] [flang-rt] Consider the signedness of zero for PRODUCT (PR #226918)

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> I forgot to mention the performance impact.

I did not bother asking about it, because I am not too worried about the `-O0` performance. But I was under an impression that we inline `product` always, and maybe I am wrong. Are there limitations that prevent us from inlining `product`?

https://github.com/llvm/llvm-project/pull/226918