[Sol->Yul] Enabling storage pointers to local vars in inline assembly.

test/libsolidity/semanticTests/inlineAssembly/inline_assembly_storage_access_local_var.sol

@@ -0,0 +1,18 @@
+contract C {
+    uint256[] public a;
+
+    function f() public returns (uint256) {
+        uint256[] storage x = a;
+        uint256 off;
+        assembly {
+            sstore(x.slot, 7)
+            off := x.offset
+        }
+        assert(off == 0);
+        return a.length;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 7