SamplePGO - Add optimization reports.

This adds a couple of optimization remarks to the SamplePGO
transformation. When it decides to inline a hot function (to mimic the
inline stack and repeat useful inline decisions in the original build).

It will also report branch destinations. For instance, given the code
fragment:

     6      if (i < 1000)
     7        sum -= i;
     8      else
     9        sum += -i * rand();

If the 'else' branch is taken most of the time, building this code with
-Rpass=sample-profile will produce:

a.cc:9:14: remark: most popular destination for conditional branches at small.cc:6:9 [-Rpass=sample-profile]
      sum += -i * rand();
             ^

llvm-svn: 251330