[jdk21u-dev] RFR: 8309978: [x64] Fix useless padding

[Satyen Subramaniam]

Backporting JDK-8309978: [x64] Fix useless padding. Adjusting code to remove possibility to useless padding in code generation and adding test. Ran GHA Sanity Checks, and local Tier 1, and Tier 2 tests. Patch is clean.

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Commit messages:
 - Backport 0038491abda51b8bd39fabed53624c10abcfe077

Changes: https://git.openjdk.org/jdk21u-dev/pull/1491/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1491&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309978
  Stats: 96 lines in 4 files changed: 93 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1491.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1491/head:pull/1491

PR: https://git.openjdk.org/jdk21u-dev/pull/1491