RFR: 8341893: AArch64: Micro-optimize compressed ptr decoding

[Aleksey Shipilev]

See the bug for full description. I think there is an accidental inefficiency in decoding: we don't have to shift when shift is zero, but we are tripped by `d != s` check. Other architectures seem to do this right. This fix surgically fixes the decoding. I have a variant of a more comprehensive MacroAssembler fix that could cover this shift-to-mov translation wholesale, but that fix is riskier.

Additional testing:
 - [ ] Linux AArch64 server fastdebug, `tier{1,2,3}`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21443/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21443&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341893
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21443.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21443/head:pull/21443

PR: https://git.openjdk.org/jdk/pull/21443