[DivRemPairs] Handling for expanded-form rem - recomposition (PR42673)
While -div-rem-pairs pass can decompose rem in div+rem pair when div-rem pair
is unsupported by target, nothing performs the opposite fold.
We can't do that in InstCombine or DAGCombine since neither of those has access to TTI.
So it makes most sense to teach -div-rem-pairs about it.
If we matched rem in expanded form, we know we will be able to place div-rem pair
next to each other so we won't regress the situation.
Also, we shouldn't decompose rem if we matched already-decomposed form.
This is surprisingly straight-forward otherwise.
Reviewers: spatel, RKSimon, efriedma, ZaMaZaN4iK, bogner
Reviewed By: bogner
Subscribers: bogner, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D65298