RFR: 8326404: Assertion error when trying to compile switch with fallthrough with pattern

[Aggelos Biboudis]

The assertion error that was raised in the bug report is a result of local variables not being defined in the case of switches that mix type patterns and record patterns with fall-through. e.g., a print after the `TransPatterns` phase:


private static int run(Object o) {
  int i = 0;
  /*synthetic*/ Object selector2$temp = <*nullchk*>(o);
  /*synthetic*/ int index$3 = 0;
  switch (java.lang.runtime.SwitchBootstraps.typeSwitch(selector2$temp, index$3)) {
    case 0:
      String s;
      if (!(let s = (String)selector2$temp; in true)) {
        index$3 = 1;
        continue;
      }
      i++;
    case 1:
      {
      /*synthetic*/ Object selector4$temp = $b$O.a(); // $b$O appears undefined here 
      ...
      }
    ...
  }
}


The code for unrolling a case label into local variable declaration statements appears in `handleSwitch` here: https://github.com/openjdk/jdk/pull/17981/files#diff-e50bbfa8783f3bc8f5542452740b78f3167bee19be7365a87da2298c6333cca6L593-L606

While`patchCompletingNormallyCases` takes care of statement block propagation, for code that completes normally, unfortunately this code is called late (after the optimization that merges common type tests e.g., `case R(...): .... case R(...): ....`). 

The fix bails out of this optimization in the presence of fall-through. For example in the `run3_break1` included, the optimization is triggered. In the rest, not.

Additionally, this patch elides the unnecessary binding generation of unnamed pattern variables from the binding context.

Thanks @lahodaj for contributing this solution 🥇

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

Commit messages:
 - 8326404: Assertion error when trying to compile switch with fallthrough with pattern

Changes: https://git.openjdk.org/jdk/pull/17981/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17981&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326404
  Stats: 171 lines in 2 files changed: 165 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17981.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17981/head:pull/17981

PR: https://git.openjdk.org/jdk/pull/17981