InstanceKlass:find_method_index regression after JDK-8231610

JDK-8231610 refactored InstanceKlass::binary_search to ::quick_search, which now has a linear slow-path version in case the VM is currently dumping the dynamic CDS archive. This cause the methods to grow enough that ::quick_search is not inlined into find_method_index, which cause a local regression.

Since the linear search is not as performance sensitive as the default binary search, then outlining the slow-path to a different method makes sense. Tests show that this allows the performance critical path to be completely inlined again, recuperating part of the performance regression (still adds a branch to check for whether we should do a linear search to the fast path)