compiler.cfg.write-barrier: fix soundness issue with ##copy that never came up but probably should've been fixed anyway

basis/compiler/cfg/write-barrier/write-barrier-tests.factor

@@ -91,3 +91,64 @@ IN: compiler.cfg.write-barrier.tests
         T{ ##write-barrier-imm f 1 }
     } write-barriers-step
 ] unit-test
+
+! ##copy instructions
+[
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##set-slot-imm f 3 1 }
+        T{ ##write-barrier-imm f 2 }
+    }
+] [
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##set-slot-imm f 3 1 }
+        T{ ##write-barrier-imm f 2 }
+    } write-barriers-step
+] unit-test
+
+[
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##set-slot-imm f 3 2 }
+        T{ ##write-barrier-imm f 1 }
+    }
+] [
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##set-slot-imm f 3 2 }
+        T{ ##write-barrier-imm f 1 }
+    } write-barriers-step
+] unit-test
+
+[
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##copy f 3 2 }
+        T{ ##set-slot-imm f 3 1 }
+        T{ ##write-barrier-imm f 2 }
+    }
+] [
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##copy f 3 2 }
+        T{ ##set-slot-imm f 3 1 }
+        T{ ##write-barrier-imm f 2 }
+    } write-barriers-step
+] unit-test
+
+[
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##copy f 3 2 }
+        T{ ##set-slot-imm f 4 1 }
+        T{ ##write-barrier-imm f 3 }
+    }
+] [
+    V{
+        T{ ##copy f 2 1 }
+        T{ ##copy f 3 2 }
+        T{ ##set-slot-imm f 4 1 }
+        T{ ##write-barrier-imm f 3 }
+    } write-barriers-step
+] unit-test

basis/compiler/cfg/write-barrier/write-barrier.factor

@@ -50,7 +50,7 @@ M: gc-map-insn eliminate-write-barrier
     fresh-allocations get clear-assoc ;
 
 M: ##copy eliminate-write-barrier
-    [ src>> ] [ dst>> ] bi copies get set-at t ;
+    [ src>> resolve-copy ] [ dst>> ] bi copies get set-at t ;
 
 M: insn eliminate-write-barrier drop t ;