RFR: Fix Optimized builds

[Aleksey Shipilev]

The usual trap in Hotspot code is the belief that `ASSERT <=> !PRODUCT`, but that's not actually true. You would not see this in regular release (`!ASSERT`, `PRODUCT`) or fastdebug (`ASSERT`, `!PRODUCT`) builds, but you would see it in optimized (`!ASSERT`, `!PRODUCT`) builds. See for example GHA: https://github.com/shipilev/loom/runs/3717294006?check_suite_focus=true

The use of `ASSERT` and `PRODUCT` should be consistent to avoid build failures in optimized builds.

Additional testing:
 - [x] Linux x86_64 optimized now builds
 - [x] Linux x86_64 release still builds
 - [x] Linux x86_64 fastdebug still builds

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Commit messages:
 - Consistent use for ASSERT and PRODUCT

Changes: https://git.openjdk.java.net/loom/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=69&range=00
  Stats: 15 lines in 7 files changed: 2 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/loom/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/69/head:pull/69

PR: https://git.openjdk.java.net/loom/pull/69