Remove opt-bisect support for "cases" in favor of debug counters

Summary:
Ths "cases" support was not quite finished, is unused, and is really just debug counters.
(well, almost, debug counters are slightly more powerful, in that they can skip things at the start, too).
Note, opt-bisect itself could also be implemented as a wrapper around
debug counters, but not sure it's worth it ATM.

I'll shove it on a todo list if we think it is.

Reviewers: MatzeB, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30856

llvm-svn: 297542