[PowerPC] Fix CR Bit spill pseudo expansion

The current CRBIT spill pseudo-op expansion creates a KILL instruction
that kills the CRBIT and defines the enclosing CR field. However, this
paints a false picture to the register allocator that all bits in the CR
field are killed so copies of other bits out of the field become dead and
removable.
This changes the expansion to preserve the KILL flag on the CRBIT as an
implicit use and to treat the CR field as an undef input.

Thanks to Hal Finkel for the review and Uli Weigand for implementation input.

Differential revision: https://reviews.llvm.org/D55996

llvm-svn: 350153