RFR: 8303244: G1: call CardTable::clear_MemRegion directly

[Albert Mingkun Yang]

On Mon, 27 Feb 2023 13:06:45 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple refactoring to skip a guard in card-table clearing.
> 
> Test: hotspot_gc

`(not all of which must be covered.)` for `clear_MemRegion` is inaccurate. I will fix that in another PR.

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PR: https://git.openjdk.org/jdk/pull/12764