function to check for stable reference?

[Jason Merrill]

Some places in the code check TREE_SIDE_EFFECTS to determine whether or not
it is safe to combine multiple evaluations of the expression.  If the expr
has already been passed to stabilize_reference it is safe, because any
subexpressions with side effects will only be evaluated once anyway.  Is
there a function to check for this?  A quick grep didn't turn up anything.

Jason