RFR: 8297830: aarch64: Make Address a descriminated union internally

[Kim Barrett]

Please review this change to the aarch64 Address class.  It now uses an
internal union, separating the literal and nonliteral cases.

This avoids leaving some fields uninitialized or initializing them to dummy
values.  It also reduces the size of the Address class somewhat, though it's
unclear whether that makes any noticeable difference.

Testing:
mach5 tier1 for linux-aarch64 and macosx-aarch64
mach5 tier2-5 for linux-aarch64

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Commit messages:
 - unionize

Changes: https://git.openjdk.org/jdk/pull/11429/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11429&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297830
  Stats: 205 lines in 3 files changed: 110 ins; 16 del; 79 mod
  Patch: https://git.openjdk.org/jdk/pull/11429.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11429/head:pull/11429

PR: https://git.openjdk.org/jdk/pull/11429