Add extra token for assembly assignment

Adding an extra token for := prevents whitespace between : = being valid

test/libsolidity/syntaxTests/inlineAssembly/invalid/assign_to_instruction.sol

@@ -0,0 +1,10 @@
+contract C {
+  function f() public pure {
+    assembly {
+      let mod := 2
+    }
+  }
+}
+// ----
+// ParserError: (67-70): Cannot use instruction names for identifier names.
+// ParserError: (71-73): Expected ';' but got ':='

test/libsolidity/syntaxTests/inlineAssembly/invalid/missing_variable_in_assign.sol

@@ -7,5 +7,5 @@ contract C {
   }
 }
 // ----
-// ParserError: (87-88): Literal, identifier or instruction expected.
-// ParserError: (87-88): Expected primary expression.
+// ParserError: (87-89): Literal, identifier or instruction expected.
+// ParserError: (87-89): Expected primary expression.

test/libsolidity/syntaxTests/inlineAssembly/invalid/multiple_assign_to_instruction.sol

@@ -0,0 +1,11 @@
+contract C {
+  function f() public pure {
+    assembly {
+      function g() -> a,b, c {}
+      let a, sub, mov := g()
+    }
+  }
+}
+// ----
+// ParserError: (102-105): Cannot use instruction names for identifier names.
+// ParserError: (105-106): Expected ';' but got ','

test/libsolidity/syntaxTests/inlineAssembly/invalid/no_identifier_assignment.sol

@@ -0,0 +1,10 @@
+contract C {
+  function f() public pure {
+    assembly {
+      return := 1
+    }
+  }
+}
+// ----
+// ParserError: (70-72): Variable name must precede ":=" in assignment.
+// ParserError: (70-72): Expected primary expression.

test/libsolidity/syntaxTests/inlineAssembly/invalid/no_identifier_label.sol

@@ -0,0 +1,10 @@
+contract C {
+  function f() public pure {
+    assembly {
+      return : 1
+    }
+  }
+}
+// ----
+// ParserError: (70-71): Label name must precede ":".
+// ParserError: (70-71): Expected primary expression.

test/libsolidity/syntaxTests/inlineAssembly/invalid/whitespace_in_assignment.sol

@@ -0,0 +1,10 @@
+contract C {
+  function f() public pure {
+    assembly {
+      let x : = mload(0)
+    }
+  }
+}
+// ----
+// ParserError: (69-70): Literal, identifier or instruction expected.
+// ParserError: (69-70): Expected primary expression.

test/libsolidity/syntaxTests/inlineAssembly/invalid/whitespace_in_multiple_assignment.sol

@@ -0,0 +1,11 @@
+contract C {
+  function f() public pure {
+    assembly {
+      function g() -> a,b, c {}
+      let x, y ,z : = g()
+    }
+  }
+}
+// ----
+// ParserError: (107-108): Literal, identifier or instruction expected.
+// ParserError: (107-108): Expected primary expression.