Codegen: MachineBlockPlacement Improve probability layout.

The following pattern was being layed out poorly:

              A
             / \
            B   C
           / \ / \
          D   E   ? (Doesn't matter)

Where A->B is far more likely than A->C, and prob(B->D) = prob(B->E)

The current algorithm gives:
A,B,C,E (D goes on worklist)

It does this even if C has a frequency count of 0. This patch
adjusts the layout calculation so that if freq(B->E) >> freq(C->E)
then we go ahead and layout E rather than C. Fallthrough half the time
is better than fallthrough never, or fallthrough very rarely. The
resulting layout is:

A,B,E, (C and D are in a worklist)

llvm-svn: 277187