[analyzer] exploded-graph-rewriter: Improve program point dumps.

- Take advantage of the stmt_point_kind.
- Dump block IDs for BlockEntrance nodes.
- Don't dump huge compound statements on PurgeDeadSymbols nodes.
- Rename Edge to BlockEdge for consistency.
- Tweak colors.

Differential Revision: https://reviews.llvm.org/D64051

llvm-svn: 364881