RFR: 8331421: ubsan: vmreg.cpp checking error member call on misaligned address

[Martin Doerr]

As shown in the JBS issue, the Undefined Behavior Sanitizer complains about `VMRegImpl::stack_0()->value()`. This can easily be avoided by skipping addition and subtraction of `first()` which is a NOP.

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Commit messages:
 - 8331421: ubsan: vmreg.cpp checking error member call on misaligned address

Changes: https://git.openjdk.org/jdk/pull/19022/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19022&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331421
  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19022.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19022/head:pull/19022

PR: https://git.openjdk.org/jdk/pull/19022