Initialize the only-used-with-PPC-double-double parts of the APFloat class. This

Initialize the only-used-with-PPC-double-double parts of the APFloat class. This
makes valgrind stop complaining about uninitialized variables being read when it
accesses a bitfield (category) that shares its bits with these variables.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127871 91177308-0d34-0410-b5e6-96231b3b80d8