flow: handle POST_OPs correctly

flow: handle POST_OPs correctly

Basically we put the POST_OPs onto a stack for processing later when we
hit a sequence point.  In C there are 5 kinds of sequence points.
1) The end of a statement
2) A comma
3) After we finish processing the arguments to a function call
4) When we hit an || or an &&
5) At the end of a condition

So at those points then we process the POST_OPs and empty the POST_OP
stack.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>