[DebugInfo] Fix implicit deref on arithmetic expressions

Author: Snowy1803

docs/HowToUpdateDebugInfo.md

@@ -206,7 +206,7 @@ debug_value %0 : $Builtin.Int64, var, name "next", type $Int, expr op_consts:1:o
 ```
 
 > [!Caution]
-> This currently doesn't work, an implicit op_deref is added.
+> This currently doesn't work if a fragment is present.
 
 #### Constants
 
@@ -219,7 +219,7 @@ debug_value undef : $Int, let, name "x", expr op_consts:1:op_fragment:#Int._valu
 ```
 
 > [!Caution]
-> This currently doesn't work, an implicit op_deref is added.
+> This currently doesn't work, these variables are dropped by IRGen.
 
 ### Undef variables
 

lib/IRGen/IRGenDebugInfo.cpp

@@ -3138,6 +3138,9 @@ bool IRGenDebugInfoImpl::buildDebugInfoExpression(
       return false;
     }
   }
+  if (Operands.size() && Operands.back() != llvm::dwarf::DW_OP_deref) {
+    Operands.push_back(llvm::dwarf::DW_OP_stack_value);
+  }
   return true;
 }
 

test/DebugInfo/irgen_undef.sil

@@ -12,4 +12,20 @@ debug_value undef : $Builtin.Int64, var, name "optimizedout"
 return %0 : $Builtin.Int64
 }
 
+// CHECK-LABEL: define {{.*}} @arithmetic
+sil @arithmetic : $@convention(thin) (Builtin.Int64) -> (Builtin.Int64) {
+bb0(%0 : $Builtin.Int64):
+// CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[CURRENT_VAR:[0-9]+]], metadata !DIExpression())
+debug_value %0 : $Builtin.Int64, var, name "current", type $Int, expr op_fragment:#Int._value
+// FIXME: It should work with the fragment, as it should be noop.
+// CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[PREVIOUS_VAR:[0-9]+]], metadata !DIExpression(DW_OP_consts, 1, DW_OP_minus, DW_OP_stack_value))
+debug_value %0 : $Builtin.Int64, var, name "previous", type $Int, expr op_consts:1:op_minus //:op_fragment:#Int._value
+// CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[NEXT_VAR:[0-9]+]], metadata !DIExpression(DW_OP_constu, 12, DW_OP_plus, DW_OP_stack_value))
+debug_value %0 : $Builtin.Int64, var, name "next", type $Int, expr op_constu:12:op_plus //:op_fragment:#Int._value
+return %0 : $Builtin.Int64
+}
+
 // CHECK: ![[JUST_UNDEF_VAR]] = !DILocalVariable(name: "optimizedout"
+// CHECK: ![[CURRENT_VAR]] = !DILocalVariable(name: "current"
+// CHECK: ![[PREVIOUS_VAR]] = !DILocalVariable(name: "previous"
+// CHECK: ![[NEXT_VAR]] = !DILocalVariable(name: "next"