[jdk16] RFR: 8259061: C2: assert(found) failed: memory-writing node is not placed in its original loop or an ancestor of it

[Roberto Castañeda Lozano]

Remove assertion that is too general, that is, it can fail on compilations where C2 generates correct code otherwise.

The assertion was introduced in [JDK-8255763](https://bugs.openjdk.java.net/browse/JDK-8255763) to prevent potential miscompilations in the case of irreducible CFGs, and will be reconsidered in [JDK-8258894](https://bugs.openjdk.java.net/browse/JDK-8258894) where the general case is addressed.

Tested by building (release and debug) on different platforms.

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Commit messages:
 - JDK-8259061: C2: assert(found) failed: memory-writing node is not placed in its original loop or an ancestor of it

Changes: https://git.openjdk.java.net/jdk16/pull/101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=101&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259061
  Stats: 20 lines in 1 file changed: 0 ins; 20 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/101.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/101/head:pull/101

PR: https://git.openjdk.java.net/jdk16/pull/101