[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::AssembleAcc:
     return "llvm.ppc.mma.assemble.acc";
   case MMAOp::AssemblePair:
     return "llvm.ppc.vsx.assemble.pair";
   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:
     return genMmaVqFuncType(context, /*Quad*/ 0, /*Pair*/ 0, /*Vector*/ 4);
   case MMAOp::AssemblePair:
     return genMmaVpFuncType(context, /*Quad*/ 0, /*Pair*/ 0, /*Vector*/ 2);
   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()};
   mlir::FunctionType intrFuncType{getMmaIrFuncType(context, IntrId)};
   mlir::func::FuncOp funcOp{
       builder.addNamedFunction(loc, getMmaIrIntrName(IntrId), intrFuncType)};