[jdk8u-dev] RFR: 6899049: G1: Clean up code in ptrQueue.[ch]pp and ptrQueue.inline.hpp

[Sun Jianye]

Hi all,

This pull request contains a backport of commit [b256989eb34a32c8f03be448c0645baeb5192a01](https://github.com/openjdk/jdk11u-dev/commit/b256989eb34a32c8f03be448c0645baeb5192a01) from the [openjdk/jdk11u-dev](https://github.com/openjdk/jdk11u-dev) repository.

As reported by issue :  https://bugs.openjdk.org/browse/JDK-8316278 .  We found  the indexing method of PtrQueue's buf  is not  correct  when  converting an integer of type size_t to type int, then calling the method PtrQueue::byte_index_to_index . 
The key problem is this way of using: 

size_t i=0;    _buf[byte_index_to_index((int)i)] = NULL;  

The variable i of size_t type  cannot be converted directly to an int type . Other than that, the return value of the function byte_index_to_index is the index of the array _buf, and it should be non-negative. So it  should be a type of size_t. 
Currently we have found 2 issues related to this problem,  https://bugs.openjdk.org/browse/JDK-8308169 and  https://bugs.openjdk.org/browse/JDK-8303961.  They are all triggered by a special  size number of buf, like '-XX:G1UpdateBufferSize=512M'  or  '-XX:G1SATBBufferSize=500m'
We also added a test case.
Please review this PR. Thanks.

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Commit messages:
 - 8316278: Fix the indexing method of PtrQueue's buf when a large integer value overflow

Changes: https://git.openjdk.org/jdk8u-dev/pull/374/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=374&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6899049
  Stats: 243 lines in 7 files changed: 133 ins; 42 del; 68 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/374.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/374/head:pull/374

PR: https://git.openjdk.org/jdk8u-dev/pull/374