[Support] UnusedOpPruner shouldn't erase block arguments

[uenoku]

UnusedOpPruner removes block arguments but it's not generally legal, e.g. HW module requires the module type is consistent with block arguments

circt/include/circt/Support/UnusedOpPruner.h

Lines 69 to 87 in 9e689e9

	while (!blockArgsToEraseIfUnused.empty()) {
	auto it = blockArgsToEraseIfUnused.begin();
	auto arg = *it;
	blockArgsToEraseIfUnused.erase(it);
	if (!arg.use_empty())
	continue;
	if (!llvm::all_of(arg.getOwner()->getUses(), [](auto &blockOperand) {
	return isa<BranchOpInterface>(blockOperand.getOwner());
	}))
	continue;
	unsigned argIdx = arg.getArgNumber();
	for (auto &blockOperand : arg.getOwner()->getUses()) {
	auto branchOp = cast<BranchOpInterface>(blockOperand.getOwner());
	auto operands =
	branchOp.getSuccessorOperands(blockOperand.getOperandNumber());
	eraseLaterIfUnused(operands[argIdx]);
	operands.erase(argIdx);
	}
	arg.getOwner()->eraseArgument(argIdx);