Support referencing other constants in inline assembly

test/libsolidity/syntaxTests/inlineAssembly/constant_array.sol

@@ -7,4 +7,4 @@ contract C {
     }
 }
 // ----
-// TypeError: (115-116): Only direct number constants are supported by inline assembly.
+// TypeError: (115-116): Only direct number constants and references to such constants are supported by inline assembly.

test/libsolidity/syntaxTests/inlineAssembly/constant_bytes_ref.sol

@@ -0,0 +1,11 @@
+contract C {
+    bytes32 constant x = keccak256("abc");
+    bytes32 constant y = x;
+    function f() public pure returns (uint t) {
+        assembly {
+            t := y
+        }
+    }
+}
+// ----
+// TypeError: (168-169): Only direct number constants and references to such constants are supported by inline assembly.

test/libsolidity/syntaxTests/inlineAssembly/constant_ref.sol

@@ -8,4 +8,3 @@ contract C {
     }
 }
 // ----
-// TypeError: (134-135): Only direct number constants are supported by inline assembly.