add optimization pass that removes unused qir calls.

#include "Quantum/QuantumOps.h"

#include "mlir/Dialect/StandardOps/IR/Ops.h"

#include "mlir/IR/Matchers.h"

#include "mlir/IR/PatternMatch.h"

#include "mlir/Pass/Pass.h"

#include "mlir/Pass/PassManager.h"

#include "mlir/Target/LLVMIR.h"

#include "mlir/Transforms/DialectConversion.h"

#include "mlir/Transforms/Passes.h"

namespace qcor {

class RemoveUnusedExtractQubitCalls

    : public mlir::OpRewritePattern<mlir::quantum::ExtractQubitOp> {

 public:

  RemoveUnusedExtractQubitCalls(mlir::MLIRContext* context)

      : OpRewritePattern<mlir::quantum::ExtractQubitOp>(context,

                                                        /*benefit=*/10) {}

  mlir::LogicalResult matchAndRewrite(

      mlir::quantum::ExtractQubitOp op,

      mlir::PatternRewriter& rewriter) const override {

    // Qubits returned from q.extract have exactly 

    // one user, the next ValueSemanticsInstOp. If 

    // it does not have exactly one, then it has 0, so 

    // lets remove it as the value is not even used.

    auto qubit = op.qbit();

    if (!qubit.hasOneUse()) {

      rewriter.eraseOp(op);

      return success();

    }

    return failure();

  }

};

class RemoveUnusedQallocCalls

    : public mlir::OpRewritePattern<mlir::quantum::QallocOp> {

 public:

  RemoveUnusedQallocCalls(mlir::MLIRContext* context)

      : OpRewritePattern<mlir::quantum::QallocOp>(context,

                                                  /*benefit=*/10) {}

  mlir::LogicalResult matchAndRewrite(

      mlir::quantum::QallocOp op,

      mlir::PatternRewriter& rewriter) const override {

    // QallocOp returns a qubit array, if it 

    // only has one user, then that user must be 

    // the dealloc op, so we can remove both of them

    auto qubits = op.qubits();

        auto next_op = (*qubits.user_begin());

    if (dyn_cast_or_null<mlir::quantum::DeallocOp>(next_op) &&

            qubits.hasOneUse()) {

      rewriter.eraseOp(op);

      rewriter.eraseOp(next_op);

      return success();

    }

    return failure();

  }

};

}  // namespace qcor