[BPI] Use metadata info before any other heuristics

Metadata potentially is more precise than any heuristics we use, so
it makes sense to use first metadata info if it is available. However it makes
sense to examine it against other strong heuristics like unreachable one.
If edge coming to unreachable block has higher probability then it is expected 
by unreachable heuristic then we use heuristic and remaining probability is
distributed among other reachable blocks equally.

An example where metadata might be more strong then unreachable heuristic is
as follows: it is possible that there are two branches and for the branch A
metadata says that its probability is (0, 2^25). For the branch B
the probability is (1, 2^25).
So the expectation is that first edge of B is hotter than first edge of A
because first edge of A did not executed at least once.
If first edge of A points to the unreachable block then using the unreachable
heuristics we'll set the probability for A to (1, 2^20) and now edge of A
becomes hotter than edge of B.
This is unexpected behavior.

This fixed the biggest part of https://bugs.llvm.org/show_bug.cgi?id=32214

Reviewers: sanjoy, junbuml, vsk, chandlerc

Reviewed By: chandlerc

Subscribers: llvm-commits, reames, davidxl

Differential Revision: https://reviews.llvm.org/D30631

llvm-svn: 300440