Handle spilling the PPC GPRC_NOR0 register class

GPRC_NOR0 is not a subclass of GPRC (because it also contains the ZERO pseudo
register). As a result, we also need to check for it in the spilling code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200288 91177308-0d34-0410-b5e6-96231b3b80d8