Fix ArgumentOutOfRangeException in CopyPropagation while decompiling System.Security.Cryptography.CryptoStream.ReadAsyncInternal from mscorlib.v4

ICSharpCode.Decompiler/IL/Transforms/CopyPropagation.cs

@@ -51,7 +51,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 						// dead store to stack
 						if (copiedExpr.Flags == InstructionFlags.None) {
 							// no-op -> delete
-							context.Step("remove dead store to stack: no-op -> delete", block.Instructions[i - 1]);
+							context.Step("remove dead store to stack: no-op -> delete", block.Instructions[i]);
 							block.Instructions.RemoveAt(i--);
 						} else {
 							// evaluate the value for its side-effects