Reapply "blockfreq: Rewrite BlockFrequencyInfoImpl"

This reverts commit r206677, reapplying my BlockFrequencyInfo rewrite.

I've done a careful audit, added some asserts, and fixed a couple of
bugs (unfortunately, they were in unlikely code paths).  There's a small
chance that this will appease the failing bots [1][2].  (If so, great!)

If not, I have a follow-up commit ready that will temporarily add
-debug-only=block-freq to the two failing tests, allowing me to compare
the code path between what the failing bots and what my machines (and
the rest of the bots) are doing.  Once I've triggered those builds, I'll
revert both commits so the bots go green again.

[1]: http://bb.pgr.jp/builders/ninja-x64-msvc-RA-centos6/builds/1816
[2]: http://llvm-amd64.freebsd.your.org/b/builders/clang-i386-freebsd/builds/18445

<rdar://problem/14292693>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206704 91177308-0d34-0410-b5e6-96231b3b80d8
blockfreq: Temporarily turn on -debug-only=block-freq

These tests fail after my BlockFrequencyInfo rewrite on two buildbots
[1][2].  I can't reproduce it locally, so I'm temporarily turning on
-debug-only=block-freq so I can find the problem.

[1]: http://bb.pgr.jp/builders/ninja-x64-msvc-RA-centos6/builds/1860
[2]: http://llvm-amd64.freebsd.your.org/b/builders/clang-i386-freebsd/builds/18477

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206705 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "blockfreq: Temporarily turn on -debug-only=block-freq"

This reverts commit r206705, as planned.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206706 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "blockfreq: Rewrite BlockFrequencyInfoImpl"

This reverts commit r206704, as expected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206707 91177308-0d34-0410-b5e6-96231b3b80d8