The LRG's _area value 0. is used by RA to avoid additional splits
of a spill copy which has one immediately following use.
But _area value 0. could be also produced if a block has frequency 0.
caused by a long chain of conditional branches with small
probability P <= 0.1 so that a float value underflow.
As result all LRGs in such blocks have the same high score 1e35
and RA can not make progress producing a lot of spill copies.
Set minimum block frequency MIN_BLOCK_FREQUENCY 1.e-35f.
Add the assert to check a block frequency.