RFR: 8377726: Remove unused macros in register.hpp

[Kim Barrett]

Please review this change to remove some unused (or nearly unused) macros in
share/asm/register.hpp. This change also simplifies the zero definition of
noreg to render one of the macros no longer used.

Testing: mach5 tier1 and GHA Sanity Tests.
The GHA testing failed to build on Alpine Linux because of the unrelated
https://bugs.openjdk.org/browse/JDK-8377707

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Commit messages:
 - remove unused macros
 - simplify noreg definition for zero

Changes: https://git.openjdk.org/jdk/pull/29685/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29685&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377726
  Stats: 22 lines in 2 files changed: 0 ins; 18 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/29685.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29685/head:pull/29685

PR: https://git.openjdk.org/jdk/pull/29685