Reapply r185393.

Original commit message:

Remove floating point computations from SpillPlacement.cpp.

Patch by Benjamin Kramer!

Use the BlockFrequency class instead of floats in the Hopfield network
computations. This rescales the node Bias field from a [-2;2] float
range to two block frequencies BiasN and BiasP pulling in opposite
directions. This construct has a more predictable behavior when block
frequencies saturate.

The per-node scaling factors are no longer necessary, assuming the block
frequencies around a bundle are consistent.

This patch can cause the register allocator to make different spilling
decisions. The differences should be small.
Remove floats from live range splitting costs.

These floats all represented block frequencies anyway, so just use the
BlockFrequency class directly.

Some floating point computations remain in tryLocalSplit(). They are
estimating spill weights which are still floats.