RFR: 8292203: AArch64: Represent Registers as values

[Vladimir Ivanov]

(It's a PR dependent on #9815.)

Along with x86 ([JDK-8292153](https://bugs.openjdk.org/browse/JDK-8292153)), I propose to refactor Register classes on AArch64 and migrate away from pointer-based representation.

The motivation is the same: improve compile-time checks and avoid ambiguities between overloads due to implicit conversions between pointers and integral types.

Proposed change hides pointer representation behind value class.

Pointer-based representation is kept to avoid massive migration of users (from `->` to `.`) and to enable incremental migration on per-platform basis (pointer-based representation is assumed in shared code).

Code quality improves:
  GCC:   https://godbolt.org/z/r6G36facj
  Clang: https://godbolt.org/z/x5oPdYEPM

(I noticed one downside: slowdebug builds become slower, because `operator->` isn't inlined there. If it becomes a problem, migrating performance-sensitive places from `->` to `.` should solve the problem.)

Testing: hs-tier1 - hs-tier5

PS: a number of cleanups are incorporated. In particular, I decided to expand all macros from `register.hpp` because IMO they add confusion rather than improve readability.

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Commit messages:
 - 8292203: AArch64: Represent Registers as values
 - cleanups
 - x86: Register class wrappers

Changes: https://git.openjdk.org/jdk/pull/9826/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9826&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292203
  Stats: 1102 lines in 43 files changed: 273 ins; 156 del; 673 mod
  Patch: https://git.openjdk.org/jdk/pull/9826.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9826/head:pull/9826

PR: https://git.openjdk.org/jdk/pull/9826