Codegen: [PPC] Silence false-positive initialization warning. NFC

Some compilers don't do exhaustive switch checking. For those compilers,
add an initialization to prevent un-initialized variable warnings from
firing. For compilers with exhaustive switch checking, we still get a
guarantee that the switch is exhaustive, and hence the initializations
are redundant, and a non-functional change.

llvm-svn: 257923