Mark the PPC CTRRC and CTRRC8 register classes as non-allocatable.

Marking these classes as non-alocatable allows CTR loop generation to
work correctly with the block placement passes, etc. These register
classes are currently used only by some unused TCRETURN patterns.
In future cleanup, these will be removed.

Thanks again to Jakob for suggesting this fix to the CTR loop problem!

llvm-svn: 158221