Do not allocate memory objects if they will be assigned directly.

test/libsolidity/semanticTests/memoryManagement/struct_allocation.sol

@@ -0,0 +1,24 @@
+contract C {
+    struct S { uint x; uint y; uint z; }
+    function memorySize() internal pure returns (uint s) {
+        assembly { s := mload(0x40) }
+    }
+    function withValue() public pure returns (uint) {
+        S memory x = S(1, 2, 3);
+        uint memorySizeBefore = memorySize();
+        S memory t = x;
+        uint memorySizeAfter = memorySize();
+        return memorySizeAfter - memorySizeBefore;
+    }
+    function withoutValue() public pure returns (uint) {
+        uint memorySizeBefore = memorySize();
+        S memory t;
+        uint memorySizeAfter = memorySize();
+        return memorySizeAfter - memorySizeBefore;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// withValue() -> 0x00
+// withoutValue() -> 0x60