clean bool Compiler::fgRemoveDeadStore

[sandreenko]

For example:

    if (asgNode->OperIs(GT_ASG))
    {
        rhsNode = asgNode->gtGetOp2();
    }
    else if (rhsNode == nullptr) <- does not make sense. does it?
    {
        return false;
    }

    if (asgNode && (asgNode->gtFlags & GTF_ASG))
    {
        noway_assert(rhsNode);
        noway_assert(tree->gtFlags & GTF_VAR_DEF);

        assert(asgNode->OperIs(GT_ASG)); <- so in previous condition there is no need in else.