Mark most PPC register classes to avoid write-after-write.

For processors with the G5-like instruction-grouping scheme, this helps avoid
early group termination due to a write-after-write dependency within the group.
It should also help on pipelined embedded cores.

On POWER7, over the test suite, this gives an average 0.5% speedup. The largest
speedups are:

SingleSource/Benchmarks/Stanford/Quicksort - 33%
MultiSource/Applications/d/make_dparser - 21%
MultiSource/Benchmarks/FreeBench/analyzer/analyzer - 12%
MultiSource/Benchmarks/MiBench/telecomm-FFT/telecomm-fft - 12%

Largest slowdowns:

SingleSource/Benchmarks/Stanford/Bubblesort - 23%
MultiSource/Benchmarks/Prolangs-C++/city/city - 21%
MultiSource/Benchmarks/BitBench/uuencode/uuencode - 16%
MultiSource/Benchmarks/mediabench/mpeg2/mpeg2dec/mpeg2decode - 13%

llvm-svn: 158719