6750588: assert(lrg._area >= 0,"negative spill area") running NSK stmp0101 test
Summary: handle NaN costs more carefully
Reviewed-by:

In extreme cases not found in normal programs, an estimated block
frequency can become the float value Infinity. Calculations for cost
and area estimates in ifg_physical() can then become Inf or NaN.
Register allocation still behaves OK with these values, but asserts
will trigger without need.

The fix changes the cost calculation to not become NaN in the face of
an Inf frequency. Two asserts are tweaked to properly check for
negative values.