Add 'StressIGVN' option to let C2 randomize IGVN worklist order. When enabled,
the worklist is shuffled before each main run of the IGVN loop. Also add
'StressSeed=N' option to specify the seed. If the seed is not specified, a
random one is generated. In either case, the seed is logged if 'LogCompilation'
is enabled. The new options are declared as production+diagnostic for
consistency with the existing 'StressLCM' and 'StressGCM' options.
Reviewed-by: kvn, chagedorn, thartmann
