Modify the ppc backend to use two register classes for FP: F8RC and F4RC.

Modify the ppc backend to use two register classes for FP: F8RC and F4RC.
These are used to represent float and double values, and the two regclasses
contain the same physical registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23577 91177308-0d34-0410-b5e6-96231b3b80d8