RFR: 8310130: C2: assert(false) failed: scalar_input is neither phi nor a matchin reduction

[Emanuel Peter]

Removed a spurious assert before optimization bailout.

I assumed that the "scalar input" of a Reduction node must always be either a Phi or another Reduction node. But that is incorrect, partial vectorization can lead to a reduction node chain where we have vector reductions and scalar reductions at the same time. In those cases, we cannot move the UnorderedReductions out of the loop, so a optimization bailout is appropriate.

I assessed the other asserts in `PhaseIdealLoop::move_unordered_reduction_out_of_loop`, and I think they are all justified. However, one assert would have lead to a `continue` in production, which would not break out of the nested loop correctly. I changed it to a `return`, so that would be a bailout from the optimization. This assert should not be triggered because in `SuperWord::mark_reductions` we forbid that a reduction node has any uses inside the loop except for the successor node in the reduction chain.

I have one regression test delivered by the fuzzer, and one that I constructed myself after understanding the issue.
Testing up to tier6 and stress testing. **Running, passing except for some IR rules I had to fix, rerunning...**

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Commit messages:
 - make test restrictions tighter
 - fixed IR rule of new test
 - fix whitespace
 - fix bailout of another assert
 - improved tests
 - 8310130: C2: assert(false) failed: scalar_input is neither phi nor a matchin reduction

Changes: https://git.openjdk.org/jdk/pull/14494/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14494&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310130
  Stats: 145 lines in 3 files changed: 141 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14494.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14494/head:pull/14494

PR: https://git.openjdk.org/jdk/pull/14494