model: only backtrack fences when acquire is before release

model: only backtrack fences when acquire is before release

Because there are up to 4 actions involved in this search for a
"conflict" (load, fence-acquire, fence-release, store), I overlooked the
fact that we only need to backtrack when the release comes after acquire
in the execution order. This fix skips past the 'release' action before
we begin searching for the 'acquire'.