Add isBarrier to SDep

SDep had is* functions for the other kinds of order dependencies (isMustAlias,
isWeak, isArtificial, etc.), but not for barrier. Upcoming commits in the
PowerPC backend will make use of this function.
Improve instruction scheduling for the PPC POWER7

Aside from a few minor latency corrections, the major change here is a new
hazard recognizer which focuses on better dispatch-group formation on the
POWER7. As with the PPC970's hazard recognizer, the most important thing it
does is avoid load-after-store hazards within the same dispatch group. It uses
the POWER7's special dispatch-group-terminating nop instruction (instead of
inserting multiple regular nop instructions). This new hazard recognizer makes
use of the scheduling dependency graph itself, built using AA information, to
robustly detect the possibility of load-after-store hazards.

significant test-suite performance changes (the error bars are 99.5% confidence
intervals based on 5 test-suite runs both with and without the change --
speedups are negative):

speedups:

MultiSource/Benchmarks/FreeBench/pcompress2/pcompress2
	-0.55171% +/- 0.333168%

MultiSource/Benchmarks/TSVC/CrossingThresholds-dbl/CrossingThresholds-dbl
	-17.5576% +/- 14.598%

MultiSource/Benchmarks/TSVC/Reductions-dbl/Reductions-dbl
	-29.5708% +/- 7.09058%

MultiSource/Benchmarks/TSVC/Reductions-flt/Reductions-flt
	-34.9471% +/- 11.4391%

SingleSource/Benchmarks/BenchmarkGame/puzzle
	-25.1347% +/- 11.0104%

SingleSource/Benchmarks/Misc/flops-8
	-17.7297% +/- 9.79061%

SingleSource/Benchmarks/Shootout-C++/ary3
	-35.5018% +/- 23.9458%

SingleSource/Regression/C/uint64_to_float
	-56.3165% +/- 25.4234%

SingleSource/UnitTests/Vectorizer/gcc-loops
	-18.5309% +/- 6.8496%

regressions:

MultiSource/Benchmarks/ASCI_Purple/SMG2000/smg2000
	18.351% +/- 12.156%

SingleSource/Benchmarks/Shootout-C++/methcall
	27.3086% +/- 14.4733%