[MBP] Add flags to disable the BadCFGConflict check in MachineBlockPlacement.

Some benchmarks have shown that this could lead to a potential
performance benefit, and so adding some flags to try to help measure the
difference.

A possible explanation. In diamond-shaped CFGs (A followed by either
B or C both followed by D), putting B and C both in between A and
D leads to the code being less dense than it could be. Always either
B or C have to be skipped increasing the chance of cache misses etc.
Moving either B or C to after D might be beneficial on average.

In the long run, but we should probably do a better job of analyzing the
basic block and branch probabilities to move the correct one of B or
C to after D. But even if we don't use this in the long run, it is
a good baseline for benchmarking.

Original patch authored by Daniel Jasper with test tweaks and a second
flag added by me.

Differential Revision: http://reviews.llvm.org/D6969

llvm-svn: 226034