[DebugInfo] [Mem2Reg] Salvage removed uninhabited variables

For instance, Never.init(from:) will allocate self, and never store into it
as that is impossible, so mem2reg will remove it.

Author: Snowy1803

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -24,6 +24,7 @@
 #include "swift/Basic/GraphNodeWorklist.h"
 #include "swift/Basic/TaggedUnion.h"
 #include "swift/SIL/BasicBlockDatastructures.h"
+#include "swift/SIL/DebugUtils.h"
 #include "swift/SIL/Dominance.h"
 #include "swift/SIL/OSSALifetimeCompletion.h"
 #include "swift/SIL/Projection.h"
@@ -2124,6 +2125,14 @@ bool MemoryToRegisters::promoteAllocation(AllocStackInst *alloc,
   // Remove write-only AllocStacks.
   if (isWriteOnlyAllocation(alloc) && !lexicalLifetimeEnsured(alloc)) {
     LLVM_DEBUG(llvm::dbgs() << "*** Deleting store-only AllocStack: "<< *alloc);
+    if (alloc->getElementType().getASTType()->isUninhabited()) {
+      if (auto VarInfo = alloc->getVarInfo())
+        SILBuilder(alloc, alloc->getDebugScope())
+          .createDebugValue(alloc->getLoc(),
+                            SILUndef::get(alloc), *alloc->getVarInfo());
+      for (Operand *use : getDebugUses(alloc))
+        use->set(SILUndef::get(alloc));
+    }
     deleter.forceDeleteWithUsers(alloc);
     return true;
   }

test/DebugInfo/mem2reg.sil

@@ -0,0 +1,15 @@
+// RUN: %target-sil-opt -enable-sil-verify-all -sil-print-debuginfo -mem2reg %s | %FileCheck %s
+sil_stage canonical
+
+import Builtin
+import Swift
+
+// CHECK-LABEL: sil {{.+}} @uninhabited
+sil [ossa] @uninhabited : $@convention(thin) () -> () {
+bb0:
+  // CHECK: debug_value undef : $*Never, var, name "self", loc "never.swift":90:10
+  %2 = alloc_stack [var_decl] $Never, var, name "self", loc "never.swift":90:10
+  dealloc_stack %2 : $*Never, loc "never.swift":90:10
+  %11 = tuple (), loc "never.swift":94:57
+  return %11 : $(), loc "never.swift":94:57
+} // end sil function '$uninhabited'