RFR: 8271896: Remove unnecessary top address checks in BOT

[Albert Mingkun Yang]

On Thu, 5 Aug 2021 07:58:12 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change of removing unnecessary top checks. I left the one `HeapRegion::block_size` as an assert, since it's outside BOT.
> 
> Test: hotspot_gc

Thanks for the review.

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PR: https://git.openjdk.java.net/jdk/pull/5013