[Polly] Fix -polly-opt-isl -analyze

The member LastSchedule was never set, such that printScop would always
print "n/a" instead of the last schedule.

To ensure that the isl_ctx lives as least as long as the stored
schedule, also store a shared_ptr.

Also set the schedule tree output style to ISL_YAML_STYLE_BLOCK to avoid
printing everything on a single line.

`opt -polly-opt-isl -analyze` will be used in the next commit.