Fix Yul codegen when dynamic array is used as rhs of assignment

test/libsolidity/semanticTests/viaYul/array_2d_assignment.sol

@@ -0,0 +1,14 @@
+contract C {
+	function f(uint n) public pure returns (uint) {
+		uint[][] memory a = new uint[][](2);
+		for (uint i = 0; i < 2; ++i)
+			a[i] = new uint[](3);
+		a[1][1] = n;
+		uint[] memory b = a[1];
+		return b[1];
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint256): 42 -> 42