[NFC] Cleanup with variable name IsPPC64 & IsDarwin

Clean up PPCAsmPrinter with IsPPC64 and IsDarwin.

Differential Revision: https://reviews.llvm.org/D69259

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375420 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed