Keeping the start state of each block around costs more than just
recomputing them as necessary, especially because many blocks only have
one predecessor and don't need any merging at all. Stop storing the
start state, and reuse predecessors' end states as much as conveniently
possible.

Change-Id: I549bad9e1a35af76a974e46fe69f74cd4dce873b
Reviewed-on: https://go-review.googlesource.com/92399Reviewed-by: David Chase <drchase@google.com>