[PowerPC] Fix gcc warning [NFC]

Without the fix gcc 7.4 warns with

../lib/Target/PowerPC/PPCAsmPrinter.cpp: In member function 'void {anonymous}::PPCAsmPrinter::EmitTlsCall(const llvm::MachineInstr*, llvm::MCSymbolRefExpr::VariantKind)':
../lib/Target/PowerPC/PPCAsmPrinter.cpp:525:53: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
                  MCInstBuilder(Subtarget->isPPC64() ? Opcode : PPC::BL_TLS)
                                ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~