Bugfix and tests.

Co-authored-by: Kamil Śliwak <kamil.sliwak@codepoets.it>

test/libsolidity/semanticTests/unused_store_storage_removal_bug.sol

@@ -0,0 +1,15 @@
+contract C {
+	uint public x;
+	function f() public {
+		x = 1; // This write used to be removed by the Yul optimizer due to the StorageWriteRemovalBeforeConditionalTermination bug.
+		g();
+		x = 2;
+	}
+	function g() internal {
+		if (msg.data.length > 4) return;
+		assembly { return(0, 0) }
+	}
+}
+// ----
+// f() ->
+// x() -> 1