Fix DenseMap::reserve(): the formula was wrong

Summary:
Just running the loop in the unittests for a few more iterations
(till 48) exhibit that the condition on the limit was not handled
properly in r263522.
Rewrite the test to use a class to count move/copies that happens
when inserting into the map.
Also take the opportunity to refactor the logic to compute the
number of buckets required for a given number of entries in the map.
Use this when constructing a DenseMap with a desired size given to
the constructor (and add a tests for this).

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18345

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 264384