RFR: 8314748: 1-10% regressions on Crypto micros

[Andrew Haley]

Performance improvement. This also reduces the delta between mainline head and the backportts in 11u and 8u.

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Commit messages:
 - Remove unused scratch reg.
 - 8314748: 1-10% regressions on Crypto micros
 - 8314748: 1-10% regressions on Crypto micros

Changes: https://git.openjdk.org/jdk/pull/15427/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15427&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314748
  Stats: 32 lines in 2 files changed: 4 ins; 0 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/15427.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15427/head:pull/15427

PR: https://git.openjdk.org/jdk/pull/15427