RFR: 8275638: GraphKit::combine_exception_states fails with "matching stack sizes" assert

[Roland Westrelin]

Root cause is identical to 8273165 AFIU: late inline of a virtual call
can throw from 2 different paths (null check and the call
itself). That breaks because the logic for exceptions expects the
stack for all paths that throw exceptions to have the same stack size.

AFAIU, the stack doesn't matter exception handling: either the
exception is caught by a exception handler and then the stack is
popped and the exception is pushed or, the exception is rethrown to
the caller in which case the current stack is also popped (that is the
jvm state for the current method). As a consequence the fix I propose
is to ignore the stack in GraphKit::combine_exception_states().

AFAIU, the same fix would work for 8273165 but I left the current work
around as is: not sure if we want to be conservative for now or not

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Commit messages:
 - more
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/6572/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6572&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275638
  Stats: 85 lines in 2 files changed: 81 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6572.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6572/head:pull/6572

PR: https://git.openjdk.java.net/jdk/pull/6572