Merge pull request #4729 from ethereum/slot-on-non-storage

Properly handle invalid references used together with _slot and _offset.

test/libsolidity/syntaxTests/inlineAssembly/storage_reference_on_function.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() pure public {
+        assembly {
+            let x := f_slot
+        }
+    }
+}
+// ----
+// TypeError: (84-90): The suffixes _offset and _slot can only be used on storage variables.