RFR: 8278289: Drop G1BlockOffsetTablePart::_object_can_span [v2]

[Albert Mingkun Yang]

> Simple change of dropping a debug-only field. Instead, the assert logic queries for the region type directly.
> 
> Test: tier1-5

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6718/files
  - new: https://git.openjdk.java.net/jdk/pull/6718/files/3f124ba2..8c8f7d02

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6718&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6718&range=00-01

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6718.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6718/head:pull/6718

PR: https://git.openjdk.java.net/jdk/pull/6718