RFR: 8256807: C2: Not marking stores correctly as mismatched in string opts

[Christian Hagedorn]

The observed assertion failure can be traced back to not marking stores correctly as mismatched when creating them in string opts. [JDK-8140390](https://bugs.openjdk.java.net/browse/JDK-8140390) actually tried to fix that but accidentally set `require_atomic_access` to true instead of `mismatched`. This change fixes that.

Thanks,
Christian

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Commit messages:
 - 8256807: C2: Not marking stores correctly as mismatched in string opts

Changes: https://git.openjdk.java.net/jdk/pull/1450/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1450&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256807
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1450.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1450/head:pull/1450

PR: https://git.openjdk.java.net/jdk/pull/1450