RFR(s): 8231630: Optimize boot loader with no bootclasspath append entry

[Jiangli Zhou]

Please review the following small optimization for boot loader.

webrev: http://cr.openjdk.java.net/~jiangli/8231630/webrev.00/
RFE: https://bugs.openjdk.java.net/browse/JDK-8231630

During classloading, the boot loader in the delegation path calls
SystemDictionary::load_instance_class to load a requested class. After
the module system is initialized, the VM checks if the requested
class' package is in a module defined to the boot loader. If the class
is either in the unnamed package or unnamed module, or in a module not
defined to the boot loader, the VM only searches the bootloader's
append entry (or entries). As a simple optimization, if there is no
boot append entry, we can immediately return NULL in that case without
any additional work. This particular call path is heavily exercised in
some microbenchmarks, such as Dacapo 'pmd'. The improvement is mostly
within noise level and may be un-noticeable for real world
applications, but does show a ~3% difference with 'pmd' in
Before/After comparison. As 'pmd' results can be noisy, the comparison
is done with a large number of benchmark runs.

Testing with submit-repo tests.

Best regards,
Jiangli