RFR: 8270540 G1: Refactor range checking in G1BlockOffsetTablePart::block_start* to asserts

[Ivan Walulya]

Hi all,

Please review this clean-up to replace range checking with asserts. 

Testing: tier 1-3

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Commit messages:
 - change to asserts

Changes: https://git.openjdk.java.net/jdk/pull/4794/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4794&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270540
  Stats: 13 lines in 1 file changed: 0 ins; 6 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4794.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4794/head:pull/4794

PR: https://git.openjdk.java.net/jdk/pull/4794