RFR: 8352681: C2 compilation hits asserts "must set the initial type just once"

[Cesar Soares Lucas]

The reason for the error reported is that when RAM tries to reduce a field load through a Phi it ends up calling `step_through_mergemem` with `_delay_transform` set to true and, since `step_through_mergemem` assumes that `_delay_transform` is `false`, it calls `igvn->transform` passing a MergeMem that has been added to the graph long ago.

I didn't opt to make `_delay_transform` false during the RAM transformations because that seemed to be a risky move, nevertheless I'll create an RFE and keep investigating that option.

Also, while working on a test case I found that C2 doesn't remove (at least not before EA steps) the `if (param != param) {...}` and I'm going to investigate that as a separate RFE.

I tested this with JTREG Tier 1-3 on Linux x86_64.

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Commit messages:
 - Patch step_through_mergemem to not re-register mergemems.

Changes: https://git.openjdk.org/jdk/pull/24471/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24471&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352681
  Stats: 72 lines in 2 files changed: 68 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/24471.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24471/head:pull/24471

PR: https://git.openjdk.org/jdk/pull/24471