RFR (M): JDK-8040722: G1: Clean up usages of heap_region_containing

[Bengt Rutisson]

Hi everyone,

Could I please have a few reviews of this cleanup?

http://cr.openjdk.java.net/~brutisso/8040722/webrev.00/

Background:

Before the perm gen was removed 
G1CollectedHeap::heap_region_containing() could return NULL for 
addresses in the perm gen. This means that a lot of code had to check 
the result since it could potentially be NULL.

Now the only reason to return NULL is if the address passed in NULL. In 
many places it is possible to know that the address is not NULL and in 
the other places it is just as simple to check the address for NULL 
before calling G1CollectedHeap::heap_region_containing() rather than 
checking for NULL after having called it.

So, to simplify the code a bit we can assert that 
G1CollectedHeap::heap_region_containing() never returns NULL and then 
only check before we call it in the places where we would otherwise have 
passed NULL to it.

Thanks,
Bengt