review for 6796786: invalid FP identity transform - (a - b) -> b - a

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/6796786

6796786: invalid FP identity transform - (a - b) -> b - a
Reviewed-by:

C2 is transforming the idiom - (a - b) into b - a which produces the
wrong sign if a == b.  The fix is to eliminate that optimization.
Tested with test from report.