[code-reflection] RFR: Unreachable method or lambda exit

[Paul Sandoz]

Incorrect code models are generated for cases where a return operation (with no operand) is inserted for an unreachable area of code. This is ambiguous when the method or lambda body returns a value.

For such cases we can instead insert an unreachable operation. Transformations can more easily ignore that.

We could take this further and replace a yield or java.continue operation with an unreachable operation e.g. for say

        {
            {
                return;
            }
            // unreachable
        }
        // unreachable
``` 
or 


        while (true) {
            while(true) {

            }
            // unreachable
        }
        // unreachable


However, in such cases there is no ambiguity regarding the yield or java.continue operation since they never accept an operand in such cases and transformations can more easily ignore them if need be. For now i have left this as is.

-------------

Commit messages:
 - Unreachable method or lambda exit.

Changes: https://git.openjdk.org/babylon/pull/257/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=257&range=00
  Stats: 228 lines in 8 files changed: 217 ins; 2 del; 9 mod
  Patch: https://git.openjdk.org/babylon/pull/257.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/257/head:pull/257

PR: https://git.openjdk.org/babylon/pull/257