[lworld] RFR: 8378244: [lworld] Use guarantee instead of assert for Load/StoreNode construction

[Tobias Hartmann]

[JDK-8364191](https://bugs.openjdk.org/browse/JDK-8364191) replaced two `ShouldNotReachHere()` with asserts. Let's use a guarantee instead to make sure we also fail in release builds and keep our static analysis tools happy.

Thanks,
Tobias

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Commit messages:
 - 8378244: [lworld] Use guarantee instead of assert for Load/StoreNode construction

Changes: https://git.openjdk.org/valhalla/pull/2136/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2136&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378244
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/2136.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2136/head:pull/2136

PR: https://git.openjdk.org/valhalla/pull/2136