RFR(s): 8079330: Circular dependency between G1CollectedHeap and G1BlockOffsetSharedArray

[Per Liden]

There's a circular include dependency between 
g1BlockOffsetTable.inline.hpp and g1CollectedHeap.inline.hpp. While 
working on a different change, which sorted the include lines in 
g1CollectedHeap.inline.hpp, this dependency was unveiled and caused a 
build error.

This patch turns the check_index() macro in 
g1BlockOffsetTable.inline.hpp into a function and moves it to 
g1BlockOffsetTable.cpp.

Webrev: http://cr.openjdk.java.net/~pliden/8079330/webrev.0/
Bug: https://bugs.openjdk.java.net/browse/JDK-8079330

/Per