Allow msize optimization only if it is not present.

test/libyul/yulOptimizerTests/unusedPruner/msize.yul

@@ -0,0 +1,12 @@
+{
+    let a := 1
+    let b := mload(10)
+    sstore(0, msize())
+}
+// ====
+// step: unusedPruner
+// ----
+// {
+//     pop(mload(10))
+//     sstore(0, msize())
+// }

test/libyul/yulOptimizerTests/unusedPruner/no_msize.yul

@@ -0,0 +1,9 @@
+{
+    let a := 1
+    let b := mload(10)
+    sstore(0, 5)
+}
+// ====
+// step: unusedPruner
+// ----
+// { sstore(0, 5) }