Skip resolving Keccak if AST contains msize.

Also a refactoring changing `m_optimizeMLoad` to `m_containsMSize`.

test/libyul/yulOptimizerTests/loadResolver/keccak_with_msize.yul

@@ -0,0 +1,19 @@
+{
+    sstore(0, msize())
+    mstore(0, 10)
+    // Keccak-256 should not be evaluated because of the msize.
+    // Even though, though the code is likely equivalent,
+    // we skip steps if `msize` is present
+    let val := keccak256(0, 32)
+    sstore(1, val)
+}
+// ----
+// step: loadResolver
+//
+// {
+//     let _1 := msize()
+//     let _2 := 0
+//     sstore(_2, _1)
+//     mstore(_2, 10)
+//     sstore(1, keccak256(_2, 32))
+// }