RFR: 8237567: Refactor G1-specific code in shared VM_CollectForMetadataAllocation

[Ivan Walulya]

Hi all,

Please review this refactoring change to move G1 specific code out of VM_CollectForMetadataAllocation. 

Testing: Tier 1-3.

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

Commit messages:
 - add INCLUDE_G1GC check
 - 8237567: Refactor G1-specific code in shared VM_CollectForMetadataAllocation

Changes: https://git.openjdk.java.net/jdk/pull/5282/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5282&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8237567
  Stats: 45 lines in 4 files changed: 17 ins; 27 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5282.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5282/head:pull/5282

PR: https://git.openjdk.java.net/jdk/pull/5282