RFR: 8296318: use-def assert: special case undetected loops nested in infinite loops [v2]

[Emanuel Peter]

> `PhaseCFG::verify` checks that the nodes scheduled into the blocks have "use-after-def".
> https://github.com/openjdk/jdk/blob/8c472e481676ed0ef475c4989477d5714880c59e/src/hotspot/share/opto/block.cpp#L1376
> 
> As long as the control flow has no loops, this should always hold.
> We make sure to not check the assert if the block-head is a `LoopNode`, and the use `n` is a `Phi`, since we may have inputs from the backedge, which would be "use-before-def" in the block-scheduling order, but that is to be expected.
> https://github.com/openjdk/jdk/blob/8c472e481676ed0ef475c4989477d5714880c59e/src/hotspot/share/opto/block.cpp#L1364
> 
> **Problem**
> This assumes that all loops are properly detected, if a loop was not detected the block-head may only be a `RegionNode`, and the assert can fail (see regression test). Why are not all loops detected?
> During `PhaseIdealLoop::build_loop_tree`, we detect all loops, but not always attach infinite loops with their sub-loops to the loop tree, and then the loop-head `Regions` are not converted to `LoopNodes` in `PhaseIdealLoop::beautify_loops`.
> This behaviour is expected, see also https://github.com/openjdk/jdk/pull/11473.
> Thus, the assert fires, but it should not.
> 
> **Solution**
> Add special casing to assert: also accept if the `Region` is in an infinite subgraph, and we are looking at a `Phi` as use (the def could be values from the backedge).

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  review suggestions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11642/files
  - new: https://git.openjdk.org/jdk/pull/11642/files/72a2f0b1..abb1416c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11642&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11642&range=00-01

  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11642.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11642/head:pull/11642

PR: https://git.openjdk.org/jdk/pull/11642