RFR: 8278482: G1: Improve HeapRegion::block_is_obj [v2]

[Albert Mingkun Yang]

> Adding assertions to `HeapRegion::block_is_obj` and its callers, and removing some dead code based on the assertions.
> 
> Test: tier1-5

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

  review

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6781/files
  - new: https://git.openjdk.java.net/jdk/pull/6781/files/d430c637..c478f5b0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6781&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6781&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6781.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6781/head:pull/6781

PR: https://git.openjdk.java.net/jdk/pull/6781