RFR: 8306997: C2: "malformed control flow" assert due to missing safepoint on backedge with a switch

[Roland Westrelin]

The assert fires because a self loop (a `Loop` whose second input is
itself) is removed by loop opts. That loop comes from a switch where
the default case is a loop head (a code shape I couldn't get javac to
produce). That `Loop` should at the very least have a `Safepoint` but
the logic at parse time only looks for backedges in the non default
cases. With that fixed, the `Loop` is no longer considered dead code.

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Commit messages:
 - test & fix

Changes: https://git.openjdk.org/jdk/pull/13688/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13688&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306997
  Stats: 106 lines in 3 files changed: 104 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13688.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13688/head:pull/13688

PR: https://git.openjdk.org/jdk/pull/13688