Add an assertion on the builder to ensure that a block is set before creating an operation

This is more friendly for the user than a raw segfault

PiperOrigin-RevId: 236504102

Author: joker-eph

mlir/lib/IR/Builders.cpp

@@ -310,6 +310,7 @@ Block *FuncBuilder::createBlock(Block *insertBefore) {
 
 /// Create an operation given the fields represented as an OperationState.
 Instruction *FuncBuilder::createOperation(const OperationState &state) {
+  assert(block && "createOperation() called without setting builder's block");
   auto *op = Instruction::create(state.location, state.name, state.operands,
                                  state.types, state.attributes,
                                  state.successors, state.numBlockLists,