RFR: 8305740: C2: add print statements to assert: Can't determine return type.

[Emanuel Peter]

I added this assert before the bailout, because it probably hides bugs. Now we have failure reports with [JDK-8305185](https://bugs.openjdk.org/browse/JDK-8305185).

It is difficult to reproduce, so I'd like to add some print statements to get at least a bit of info.

Passed tests up to tier5 and stress testing.

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Commit messages:
 - 8305740: C2: add print statements to assert: Can't determine return type.

Changes: https://git.openjdk.org/jdk/pull/13385/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13385&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305740
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13385.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13385/head:pull/13385

PR: https://git.openjdk.org/jdk/pull/13385