Fix pointer arithmetic with `nint*`/`nuint*`.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/NativeInts.cs

@@ -109,6 +109,16 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			Console.WriteLine(-i);
 		}
 
+		public unsafe int* PtrArithmetic(int* ptr)
+		{
+			return ptr + i;
+		}
+
+		public unsafe nint* PtrArithmetic(nint* ptr)
+		{
+			return ptr + u;
+		}
+
 		public object[] Boxing()
 		{
 			return new object[10] {

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1143,10 +1143,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			if (!expr.Type.IsCSharpPrimitiveIntegerType() && !expr.Type.IsCSharpNativeIntegerType()) {
 				// pointer arithmetic accepts all primitive integer types, but no enums etc.
-				StackType targetType = expr.Type.GetStackType() == StackType.I4 ? StackType.I4 : StackType.I8;
-				expr = expr.ConvertTo(
-					compilation.FindType(targetType.ToKnownTypeCode(expr.Type.GetSign())),
-					this);
+				expr = expr.ConvertTo(FindArithmeticType(expr.Type.GetStackType(), expr.Type.GetSign()), this); 
 			}
 			return expr;
 		}

ICSharpCode.Decompiler/IL/PointerArithmeticOffset.cs

@@ -36,7 +36,7 @@ namespace ICSharpCode.Decompiler.IL
 				if (mul.CheckForOverflow != checkForOverflow)
 					return null;
 				if (elementSize > 0 && mul.Right.MatchLdcI(elementSize.Value)
-					|| mul.Right.UnwrapConv(ConversionKind.SignExtend) is SizeOf sizeOf && sizeOf.Type.Equals(pointerElementType)) {
+					|| mul.Right.UnwrapConv(ConversionKind.SignExtend) is SizeOf sizeOf && NormalizeTypeVisitor.TypeErasure.EquivalentTypes(sizeOf.Type, pointerElementType)) {
 					var countOffsetInst = mul.Left;
 					if (unwrapZeroExtension) {
 						countOffsetInst = countOffsetInst.UnwrapConv(ConversionKind.ZeroExtend);