RFR: 8291466: C2: assert(false) failed: infinite loop in PhaseIterGVN::transform_old with -XX:+StressIGVN

[Pengfei Li]

Recently, an igvn infinite loop issue was reported. Git bisection shows
it appears after our JDK-8289996 patch. But after that patch is backed
out, we find similar test still fails. We have attached a jtreg case to
reproduce this issue.

The ideal graph of the problematic method has a `MulINode` multiplying a
`PhiNode` by a `ConINode`. For better optimizations, `MulINode::Ideal()`
moves the constant input to the right hand side. And `Ideal()` function
in its parent class `MulNode` has the similar logic. In some code paths,
`MulINode::Ideal()` calls `MulNode::Ideal()`. The problem here is that,
`MulINode` and `MulNode` use different ways to check constant. One calls
`type->singleton()` and the other calls `node->find_int_con(val)` which
accepts constant in `PhiNode`. So in some corner cases where a `PhiNode`
can be evaluated to a constant, the two inputs of the `MulNode` will be
swapped back and forth in `Ideal()` calls. It eventually causes the igvn
infinite loop issue.

This patch removes the `swap_edges()` logic in `MulINode` and `MulLNode`
because it's enough to do this by calling `MulNode::Ideal()`. We also do
some code cleanup in this patch as we have done in JDK-8289996.

Tested hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1.

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Commit messages:
 - 8291466: C2: assert(false) failed: infinite loop in PhaseIterGVN::transform_old with -XX:+StressIGVN

Changes: https://git.openjdk.org/jdk/pull/9820/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9820&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291466
  Stats: 95 lines in 3 files changed: 77 ins; 3 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/9820.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9820/head:pull/9820

PR: https://git.openjdk.org/jdk/pull/9820