RFR: 8366488: JVM_FindClassFromClass should assert that from class is never null

[Paul Hübner]

Hi all,

The `from_class` nullcheck  in `JVM_FindClassFromClass` is redundant, as the this is the Java mirror which cannot be null during class linking/verification [1]. Therefore, it has been refactored to be an assertion instead.

Testing: tiers 1-4 Linux (x64, AArch64), macOS (x64, AArch64), Windows (x64). As part of a separate experiment, I've also run tiers 1-6 on x64 Linux with a non-null assertion before we call `Verifier::inference_verify` (which in turn, eventually, calls this code).

[1] For reasoning, see https://bugs.openjdk.org/browse/JDK-8366488

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Commit messages:
 - Convert conditional to assertion.

Changes: https://git.openjdk.org/jdk/pull/27957/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27957&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366488
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27957.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27957/head:pull/27957

PR: https://git.openjdk.org/jdk/pull/27957