[flang] Add two llvm_unreachable() calls to new code to silence compiler warning

[klausler]

Summary

This gets our -Werror build working again.

[clementval]

Thanks Peter!

flang/lib/Optimizer/Builder/PPCIntrinsicCall.cpp

   case MMAOp::DisassembleAcc:
     return "llvm.ppc.mma.disassemble.acc";
   case MMAOp::DisassemblePair:
     return "llvm.ppc.vsx.disassemble.pair";
   }
+  llvm_unreachable("getMmaIrIntrName");
 }
 
 mlir::FunctionType getMmaIrFuncType(mlir::MLIRContext *context, MMAOp mmaOp) {
   switch (mmaOp) {
   case MMAOp::AssembleAcc:

   case MMAOp::DisassembleAcc:
     return genMmaDisassembleFuncType(context, mmaOp);
   case MMAOp::DisassemblePair:
     return genMmaDisassembleFuncType(context, mmaOp);
   }
+  llvm_unreachable("getMmaIrFuncType");
 }
 
 template <MMAOp IntrId, MMAHandlerOp HandlerOp>
 void PPCIntrinsicLibrary::genMmaIntr(llvm::ArrayRef<fir::ExtendedValue> args) {
   auto context{builder.getContext()};