There are multiple ways to define the remainder operation. Depending
on the definition, the result is either always positive or has the sign
of the dividend.
The pattern lowering `arith.remf` to LLVM assumes that the semantics
match `llvm.frem`, which seems to be reasonable. The folder, however,
is implemented via `APFloat::remainder()` which has different semantics.

This patch matches the folding behaviour to lowering behavior by using
`APFloat::mod()`, which matches the behavior of `llvm.frem` and libm's
`fmod()`. It also updates the documentation of `arith.remf` to explicitly
state that it follows the semantics of `llvm.frem`.

frem documentation: https://llvm.org/docs/LangRef.html#frem-instruction

Fix llvm#94431

Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>

Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>