RFR: 8252496: C2: Useless code in MergeMemNode::Ideal

[Emanuel Peter]

This code was there since `initial load`. At that point there was other code that still depended on this code.
The other code was removed [here](https://github.com/eme64/jdk/commit/79ad5512cfe5c4ef7d0b61616f3763c62d3427a5). This was the change set for [JDK-8243670](https://bugs.openjdk.java.net/browse/JDK-8243670).
Our code was left behind.

I removed the useless code, and ran tests to verify that nothing broke due to side-effects.

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Commit messages:
 - 8252496: C2: Useless code in MergeMemNode::Ideal

Changes: https://git.openjdk.java.net/jdk/pull/7401/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7401&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252496
  Stats: 24 lines in 1 file changed: 0 ins; 24 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7401.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7401/head:pull/7401

PR: https://git.openjdk.java.net/jdk/pull/7401