[jdk11u-dev] RFR: 8290705: StringConcat::validate_mem_flow asserts with "unexpected user: StoreI"

[Goetz Lindenmaier]

I backport this for parity with 11.0.18-oracle.

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Commit messages:
 - Backport 61e072d11c8e0cb5879bb733ed1fdd2144326bfd

Changes: https://git.openjdk.org/jdk11u-dev/pull/1376/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290705
  Stats: 122 lines in 3 files changed: 122 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1376.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1376/head:pull/1376

PR: https://git.openjdk.org/jdk11u-dev/pull/1376