Remove RegionState::AccessSummary and rewrite mergePredAccesses.

AccessSummary was storing unnecessary state in every per-block entry in the
global map. It was also making most of the code in this pass very hard to read.

Rewriting mergePredAccesses allows AccessSummary to be removed. The
new implementation also avoids unnecessary DenseMap lookups.

There is also a functional change. mergePredAccesses was clearing the
state of predecessor blocks. This isn't logical and had no explanation.

Author: atrick