[DebugInfo] [SILCombine] Fix variable for opened existential

The type of the new variable should be opened too.
If the variable has an expression modifying the type, it cannot be rewritten and is dropped.

(cherry picked from commit 5f2926c)

Author: Snowy1803

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -653,8 +653,20 @@ SILInstruction *SILCombiner::visitAllocStackInst(AllocStackInst *AS) {
   if (IEI && !OEI &&
       !IEI->getLoweredConcreteType().hasOpenedExistential()) {
     assert(!IEI->getLoweredConcreteType().isOpenedExistential());
+    Builder.setCurrentDebugScope(AS->getDebugScope());
+    auto varInfo = AS->getVarInfo();
+    if (varInfo) {
+      if (varInfo->Type == AS->getElementType()) {
+        varInfo->Type = {}; // Lower the variable's type too.
+      } else {
+        // Cannot salvage the variable, its type has changed and its expression
+        // cannot be rewritten.
+        Builder.createDebugValue(AS->getLoc(), SILUndef::get(AS), *varInfo);
+        varInfo = {};
+      }
+    }
     auto *ConcAlloc = Builder.createAllocStack(
-        AS->getLoc(), IEI->getLoweredConcreteType(), AS->getVarInfo());
+        AS->getLoc(), IEI->getLoweredConcreteType(), varInfo);
     IEI->replaceAllUsesWith(ConcAlloc);
     eraseInstFromFunction(*IEI);