[DAG] refactor related div/rem folds; NFCI

This is known incomplete and not called in the right order relative to
other folds, but that's the current behavior. I'm just trying to clean
this up before making actual functional changes to make the patch smaller.

The logic here should mimic the IR equivalents that are in InstSimplify's
simplifyDivRem().

llvm-svn: 297086