[llvm] r179478 - Document the decision to assume that the cost of floats is twice as much as integers.

[Nadav Rotem]

Hi Hal, 


> It looks like this is designed to be a throughput assumption, and while this is certainly right on Sandy Bridge, Haswell and similar, it is wrong for many other platforms (non-Intel cores such as Bulldozer, many different PowerPC cores, etc.)

Makes sense. 

> 
> Should we add some kind of getDefaultFloatThroughput() callback to handle this? Alternatively, X86TTI could return '2' for scalar floating point arithmetic operations. Does anyone have a preference?
> 

I am okay with both suggestions. 

Thanks