Implement a block placement pass based on the branch probability and

block frequency analyses. This differs substantially from the existing
block-placement pass in LLVM:

1) It operates on the Machine-IR in the CodeGen layer. This exposes much
   more (and more precise) information and opportunities. Also, the
   results are more stable due to fewer transforms ocurring after the
   pass runs.
2) It uses the generalized probability and frequency analyses. These can
   model static heuristics, code annotation derived heuristics as well
   as eventual profile loading. By basing the optimization on the
   analysis interface it can work from any (or a combination) of these
   inputs.
3) It uses a more aggressive algorithm, both building chains from tho
   bottom up to maximize benefit, and using an SCC-based walk to layout
   chains of blocks in a profitable ordering without O(N^2) iterations
   which the old pass involves.

The pass is currently gated behind a flag, and not enabled by default
because it still needs to grow some important features. Most notably, it
needs to support loop aligning and careful layout of loop structures
much as done by hand currently in CodePlacementOpt. Once it supports
these, and has sufficient testing and quality tuning, it should replace
both of these passes.

Thanks to Nick Lewycky and Richard Smith for help authoring & debugging
this, and to Jakob, Andy, Eric, Jim, and probably a few others I'm
forgetting for reviewing and answering all my questions. Writing
a backend pass is *sooo* much better now than it used to be. =D

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142641 91177308-0d34-0410-b5e6-96231b3b80d8

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