[Bitcode] Clarify that extractvalue/insertvalue constexprs are not supported

Bitcode currently cannot encode these. Throw a more obvious error
in this case, rather than failing an operand count assertion.

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -2653,6 +2653,10 @@ void ModuleBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
         Record.push_back(VE.getValueID(C->getOperand(1)));
         Record.push_back(CE->getPredicate());
         break;
+      case Instruction::ExtractValue:
+      case Instruction::InsertValue:
+        report_fatal_error("extractvalue/insertvalue constexprs not supported");
+        break;
       }
     } else if (const BlockAddress *BA = dyn_cast<BlockAddress>(C)) {
       Code = bitc::CST_CODE_BLOCKADDRESS;