ipa: Fix segmentation fault in function_summary<clone_info*>::get(cgraph_node*)
PR 97660 occurs when cgraph_node::get returns NULL, and this NULL
cgraph_node is then passed to clone_info::get. As the original assert
prior to the regressing change in r11-4587 allowed for the cgraph_node
to be NULL, clone_info::get is now only called when cgraph_node::get
returns a nonnull value.