RFR: 8269516: AArch64: Assembler cleanups

[Andrew Haley]

The core of the AArch64 assembler is unnecessarily complex, with multiple indirections from class Assembler. We can get rid of them, which makes libjvm.so faster and smaller. Also, there are some dubious casts from FloatRegister to Register which should use a proper conversion function.

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Commit messages:
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Changes: https://git.openjdk.java.net/jdk/pull/4612/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4612&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269516
  Stats: 216 lines in 5 files changed: 79 ins; 72 del; 65 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4612.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4612/head:pull/4612

PR: https://git.openjdk.java.net/jdk/pull/4612