[InstCombine] propagate "exact" through folds of div

These folds were added recently with:
6b869be8
8da2fa85
...but they didn't account for the "exact" attribute,
and that can be safely propagated:
https://alive2.llvm.org/ce/z/F_WhnR
https://alive2.llvm.org/ce/z/ft9Cgr