More robust way to get index of first stack value in optimizer

The ASM analyzer changed in 9.7 to no longer call `clearStack()` before
`newExceptionValue`. We need to get the index of the exception value,
which is always the first slot after all locals.

src/compiler/scala/tools/nsc/backend/jvm/analysis/ProdConsAnalyzer.scala

@@ -469,7 +469,9 @@ class InitialProducerSourceInterpreter extends SourceInterpreter(scala.tools.asm
   }
 
   override def newExceptionValue(tryCatchBlockNode: TryCatchBlockNode, handlerFrame: Frame[SourceValue], exceptionType: Type): SourceValue = {
-    val handlerStackTop = handlerFrame.stackTop + 1 // +1 because this value is about to be pushed onto `handlerFrame`.
+    // -1 to go from the number of locals to the (0-based) index of the last local
+    // +1 because this value is about to be pushed onto `handlerFrame`
+    val handlerStackTop = (handlerFrame.getLocals - 1) + 1
     new SourceValue(1, ExceptionProducer(tryCatchBlockNode.handler, handlerStackTop))
   }
 }