update Comment

Signed-off-by: xiehuc <xiehuc@gmail.com>
fix a potential bug: find call args range isn't op_begin -> op_end

call args range is op_begin -> op_begin + getNumArgOperands(). for
future llvm 3.5 would provide arg_operands()

Signed-off-by: xiehuc <xiehuc@gmail.com>
add a macro CYCLE_EXPR_USE_DDG to switch between DDGraph::expr() and pretty_print

Signed-off-by: xiehuc <xiehuc@gmail.com>