gencgc: Introduce a new predicate, page_ends_contiguous_block_p().

gencgc: Introduce a new predicate, page_ends_contiguous_block_p().

  * There are a number of places in gencgc where a number of
attributes of a page and possibly the subsequent page are tested
for various values.  Invariably, this is actually testing to see
if a page ends a contiguous block.

  * Extract the various tests to a new inlined predicate function,
page_ends_contiguous_block_p(), thus revealing the intent of
what's going on far better than the bare tests, and coalescing the
code to a single copy to make it easier to fix if there is a bug
in it (and there is, but this is a refactoring commit, not a
behavior change commit).