Disallow assigning more than once to immutables during the vardecl

test/libsolidity/syntaxTests/immutable/variable_declaration_already.sol

@@ -0,0 +1,10 @@
+contract C {
+    uint immutable z = 2;
+    uint immutable x = z = y = 3;
+    uint immutable y = 5;
+}
+// ----
+// TypeError 1581: (62-63): Immutable variables can only be initialized inline or assigned directly in the constructor.
+// TypeError 1574: (62-63): Immutable state variable already initialized.
+// TypeError 1581: (66-67): Immutable variables can only be initialized inline or assigned directly in the constructor.
+// TypeError 1574: (66-67): Immutable state variable already initialized.

test/libsolidity/syntaxTests/immutable/variable_declaration_value.sol

@@ -0,0 +1,6 @@
+contract C {
+    int immutable x = x = 5;
+}
+// ----
+// TypeError 1581: (35-36): Immutable variables can only be initialized inline or assigned directly in the constructor.
+// TypeError 1574: (35-36): Immutable state variable already initialized.