RFR: 8264731: Introduce InstanceKlass::method_at_itable_or_null()

[Vladimir Ivanov]

Introduce `InstanceKlass::method_at_itable_or_null()` - a non-throwing variant of `InstanceKlass::method_at_itable()` that implements interface method selection. 

As a cleanup, rewrite `InstanceKlass::method_at_itable()` on top of `InstanceKlass::method_at_itable_or_null()`.

Testing: 
* [x] hs-tier1 - hs-tier6

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Commit messages:
 - InstanceKlass::method_at_itable_or_null

Changes: https://git.openjdk.java.net/jdk/pull/3344/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3344&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264731
  Stats: 46 lines in 3 files changed: 14 ins; 9 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3344.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3344/head:pull/3344

PR: https://git.openjdk.java.net/jdk/pull/3344