[APInt] Move a comment from urem to srem where it belongs.

This comment talks about sign of the dividend and the result. I
think it belongs to the srem function.

This is the commit that added it
https://github.com/llvm/llvm-project/commit/709a820a5338ca5c4b949f2f0286f7da39f00661
and the code change with it was for srem.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140611