RFR: 8293163: G1: Rename G1HeapRegionAttr::is_humongous

[Albert Mingkun Yang]

Simple renaming to avoid confusion.

Test: build

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

Commit messages:
 - g1-rename-is-humongous

Changes: https://git.openjdk.org/jdk/pull/10097/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10097&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293163
  Stats: 33 lines in 8 files changed: 0 ins; 0 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/10097.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10097/head:pull/10097

PR: https://git.openjdk.org/jdk/pull/10097