[InstCombine] Improve profile information preservation during reassociation

In Instcombine reassociations it may be possible to reason about the probabilities, reconfigure the order of reassociation or otherwise improve the preservation of profile data. We came across a few such cases while attempting to fix profile information loss. For now, we intend to mark these as unknown and revisit them later as a follow up. Filing this issue to tag and track these cases.

Start by reviewing InstCombine reassociation handling and the profile-information cases described in the issue. Investigate where reassociation loses or marks profile data as unknown. Done would require a defined approach for preserving or reasoning about probabilities, which the issue does not yet specify.

Written by the indexing model from the issue text.