After bisecting, the guilty is:

commit 3081264dbeb5c322456b1b6b9011745ab98982a1
Author: Stanislav Mekhanoshin <Stanislav.Mekhanoshin@amd.com>
Date:   Sat Mar 11 00:29:27 2017 +0000

    [AMDGPU] Remove getBidirectionalReasonRank

    This method inverts the Reason field of a scheduling candidate.
    It does right comparison between RegCritical and RegExcess, but
    everything else is broken. In fact it can prefer less strong reason
    such as Weak over RegCritical because Weak > -RegCritical.

    The CandReason enum is properly sorted, so just remove artificial
    ranking.

    Differential Revision: https://reviews.llvm.org/D30557

    git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297536
91177308-0d34-0410-b5e6-96231b3b80d8

Presumably, it's a scheduling issue. The commit looks correct though.