[PowerPC] Add profitablilty check for conversion to mtctr loops

Add profitability checks for modifying counted loops to use the mtctr instruction.

The latency of mtctr is only justified if there are more than 4 comparisons that
will be removed as a result.  Usually counted loops are formed relatively early
and before unrolling, so most low trip count loops often don't survive.  However
we want to ensure that if they do, we do not mistakenly update them to mtctr loops.

Use CodeMetrics to ensure we are only doing this for small loops with small trip counts.

Differential Revision: https://reviews.llvm.org/D38212

llvm-svn: 315592