[llvm] [DAG] Use copysign in frem power-2 fold. (PR #91751)

[David Green via llvm-commits]

davemgreen wrote:

> > when we do not know that the input is positive (or we care about sign bits)
> 
> "**and** we care about sign bits", surely?

Sure, sounds good. The original was even more wrong, and I messed up the reference commit too.

https://github.com/llvm/llvm-project/pull/91751