Fixed 6667595: Set probability FAIR for pre-, post- loops and ALWAYS for main loop
C2 clone original loop's probability when it constructs pre-, main, post- loops.
Then C2 adds zero-trip guard for main loop with probability FAIR.
As result a main loop's probability could be half of a pre-loop's probability
and RA may generates stack spills in the main loop and not in the pre-loop.
- Fix loop's probability.
- Prevent some ideal optimizations for loop's trip counter to avoid
having two live values which could cause spilling.
- Add several ideal optimizations to improve code from a loop cloning.
- Improve a spill copy removal in post allocation.
Fix verified (y/n): y, generated code.
JPRT, refworkload, specjbb2005, specjvm2008