avoid creating temporary in genObjConstr if possible (#7032)

tests/objects/tobjconstr.nim

@@ -15,7 +15,8 @@ discard """
 (y: 678, x: 123)
 (y: 678, x: 123)
 (y: 0, x: 123)
-(y: 678, x: 123)'''
+(y: 678, x: 123)
+(y: 123, x: 678)'''
 """
 
 type
@@ -75,3 +76,7 @@ when true:
   echo b                  # (y: 0, x: 123)
   b=B(y: 678, x: 123)
   echo b                  # (y: 678, x: 123)
+  b=B(y: b.x, x: b.y)
+  echo b                  # (y: 123, x: 678)
+
+GC_fullCollect()

tests/objects/tobjconstr2.nim

@@ -1,3 +1,8 @@
+discard """
+  output: '''42
+Foo'''
+"""
+
 type TFoo{.exportc.} = object
  x:int
 
@@ -48,3 +53,5 @@ type NamedGraphic = object of Graphic2
 
 var ngr = NamedGraphic(kind: Koo, radius: 6.9, name: "Foo")
 echo ngr.name
+
+GC_fullCollect()