Support referencing other constants in inline assembly

test/libsolidity/semanticTests/inlineAssembly/constant_access_referencing.sol

@@ -0,0 +1,26 @@
+contract C {
+    uint constant a = 2;
+    uint constant aa = a;
+    uint constant aaa = aa;
+    bytes2 constant b = 0xabcd;
+    bytes2 constant bb = b;
+    bytes3 constant c = "abc";
+    bytes3 constant cc = c;
+    bytes3 constant ccc = cc;
+    bytes3 constant cccc = ccc;
+    bool constant d = true;
+    bool constant dd = d;
+    address payable constant e = 0x1212121212121212121212121212121212121212;
+    address payable constant ee = e;
+    function f() public pure returns (uint w, bytes2 x, bytes3 y, bool z, address t) {
+        assembly {
+            w := aaa
+            x := bb
+            y := cccc
+            z := dd
+            t := ee
+        }
+    }
+}
+// ----
+// f() -> 2, left(0xabcd), left(0x616263), true, 0x1212121212121212121212121212121212121212

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.