PR11353: enable probe elision optimization

PR11353: enable probe elision optimization

Previously, the compiler only issued warnings for probes with empty
handlers. With the new implementation, the compiler additionally
elides probes with empty handlers. The joining of probes to their
groups has been delayed until after this optimization is performed.

Many tests in the testsuite were built under the assumption that
probes with empty handlers are not elided. Consequently, many of
them stopped working and were fixed.

1) Removed probes with empty handlers from session.
2) Issued warnings whenever a probe is elided.
3) Added tests which check for correct probe elision.
4) Fixed tests affected by probe elision.
5) Updated NEWS.