RFR: 8319820: Use unnamed variables in the FFM implementation

[Per Minborg]

This PR proposes to use unnamed variables now that they have become available in Java.

There is also a cosmetic cleanup making a switch rake smaller and where variables are pre-calculated rather than spreading expressions around.

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Commit messages:
 - Use unnamed variables

Changes: https://git.openjdk.org/jdk/pull/16594/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16594&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319820
  Stats: 35 lines in 4 files changed: 2 ins; 11 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/16594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16594/head:pull/16594

PR: https://git.openjdk.org/jdk/pull/16594