[PPC64] Fix SUBFC8 Defs list

Fix PR27943 "Bad machine code: Using an undefined physical register".
SUBFC8 implicitly defines the CR0 register, but this was omitted in
the instruction definition.

Patch by Jameson Nash <jameson@juliacomputing.com>

Reviewers: hfinkel
Differential Revision: http://reviews.llvm.org/D20802

llvm-svn: 271425