RFR: 8331252: C2: MergeStores: handle negative shift values

[Emanuel Peter]

Somehow, I have not thought of negative shift constants, and there was no regression test for it.
The fuzzer now found a case.

**I convert the assert into a condition.**

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Commit messages:
 - the fix itself
 - 8331252

Changes: https://git.openjdk.org/jdk/pull/19001/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19001&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331252
  Stats: 26 lines in 2 files changed: 23 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19001.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19001/head:pull/19001

PR: https://git.openjdk.org/jdk/pull/19001