RFR(XS) 8232071 Avoid shared dictionary lookup when the class name is not shared

[Ioi Lam]

http://cr.openjdk.java.net/~iklam/jdk14/8232071-limit-shared-dict-lookup.v01/
https://bugs.openjdk.java.net/browse/JDK-8232071

The names of all shared classes must also be a shared Symbol. To improve 
loading
time of non-shared classes, we should avoid doing the shared dictionary 
lookup,
if class_name->is_shared() == false.

I also took the chance to re-factor the code a little.

Testing: running hs-tier{1,2} now.

Thanks
- Ioi