RFR: 8272315: Improve assert_different_registers

[Kim Barrett]

Please review this improvement to assert_different_register.

It reduces ~250 lines of repetitive code to ~10 lines, and improves the
error message on failure by printing a duplicated registers name.

Unfortunately, this isn't enough to eliminate AbstractRegister[Impl].
Although it seems to no longer be needed for the Oracle-supported platforms,
the linux-arm/ppc64le/s390 platforms all use the value() function provided
by AbstractRegisterImpl.

Testing:
tier1 on Oracle supported platforms.
built linux-arm/ppc64le/s390.

Locally changed a call to have a duplicate register and verified the error
message had a useful register name.

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Commit messages:
 - simplify assert_different_registers

Changes: https://git.openjdk.java.net/jdk/pull/5083/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5083&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272315
  Stats: 258 lines in 1 file changed: 1 ins; 245 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5083.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5083/head:pull/5083

PR: https://git.openjdk.java.net/jdk/pull/5083