RFR: 8296967: JVMCI] rationalize relationship between getCodeSize and getCode in ResolvedJavaMethod

[Doug Simon]

When `ResolvedJavaMethod.getCodeSize()` returns a value > 0, `ResolvedJavaMethod.getCode()` will return `null` if the declaring class is not linked, contrary to the intuition of most JVMCI API users.
This PR rationalizes the API such that:

ResolvedJavaMethod m = ...;
ResolvedJavaType c = m.getDeclaringClass();

assert (m.getCodeSize() >  0) == (m.getCode() != null);  // m is a non-abstract, non-native method whose declaring class is linked in the current runtime
assert (m.getCodeSize() == 0) == (m.getCode() == null);  // m is an abstract or native method
assert c.isLinked()           == (m.getCodeSize() >= 0); // m's code size will always be >= 0 if its declaring class is linked in the current runtime

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Commit messages:
 - getCodeSize and hasBytecodes in ResolvedJavaMethod now return -1 and false for concrete methods in unlinked classes (GR-41977)

Changes: https://git.openjdk.org/jdk/pull/11147/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11147&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296967
  Stats: 317 lines in 8 files changed: 232 ins; 38 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/11147.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11147/head:pull/11147

PR: https://git.openjdk.org/jdk/pull/11147