RFR (S): 8079091: Remove dictionary NULL check on common path of BlockFreeList methods

[Jungwoo Ha]

BUG: https://bugs.openjdk.java.net/browse/JDK-8079091
Webrev: http://cr.openjdk.java.net/~jwha/8079091/webrev.00/

Can someone sponsor this change?
I've seen ~3% speed up on DaCapo benchmarks. (results attached on the bug
page.)
Regardless of the speed up, I think the changes are straight-forward
improvement.

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Jungwoo Ha