Add initial support for debug counting

Summary:

We have support for bisection, and bugpoint can reduce testcases
often to a single pass. But that doesn't help reduce it to a single
transform by a single pass.  Which debug counting lets us do.

Debug counting lets you instrument a pass so that it only executes a
certain thing (rwhatever you want) after skipping it a certain time of
times, and then only does a certain number of executions before saying
"skip" again.

To make it concrete, for predicateinfo, if i instrument use renaming,
i can make it so it skips renaming the first N uses, renames the next
N, and then skips the rest.

This lets you narrow down a miscompilation to, often, a single
transformation, and then also debug it (by using the same command line
parameters).

Reviewers: chandlerc, davide, mehdi_amini

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29998

llvm-svn: 295593