RFR: 8307625: Redundant receiver null check in LibraryCallKit::generate_method_call

[Eric Nothum]

The null_check_receiver() calls in generate_method_call are redundant as all callers of generate_method_call already perform this check. For future uses of generate_method_call a new assert is introduced that fails if the caller does not null check the receiver.
Also generally null_check_receiver() should be combined with stopped(), which was not the case here.

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Commit messages:
 - 8307625: remove redundant calls of null_check_receiver() and replace them with an assert

Changes: https://git.openjdk.org/jdk/pull/14542/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14542&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307625
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14542.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14542/head:pull/14542

PR: https://git.openjdk.org/jdk/pull/14542