fixes lastRead uses the when nimvm branch (nim-lang#24834)

```nim
proc foo =
  var x = "1234"
  var y = x
  when nimvm:
    discard
  else:
    var s = x
    doAssert s == "1234"
  doAssert y == "1234"

static: foo()
foo()
```
`dfa` chooses the `nimvm` branch, `x` is misread as a last read and
`wasMoved`.

`injectDestructor` is used for codegen and is not used for vmgen. It's
reasonable to choose the codegen path instead of the `nimvm` path so the
code works for codegen. Though the problem is often hidden by
`cursorinference` or `optimizer`.

found in nim-lang#24831

(cherry picked from commit 3617d2e)

Author: ringabout

compiler/dfa.nim

@@ -439,8 +439,8 @@ proc gen(c: var Con; n: PNode) =
     genUse(c, n)
   of nkIfStmt, nkIfExpr: genIf(c, n)
   of nkWhenStmt:
-    # This is "when nimvm" node. Chose the first branch.
-    gen(c, n[0][1])
+    # This is "when nimvm" node. Chose the second branch.
+    gen(c, n[1][0])
   of nkCaseStmt: genCase(c, n)
   of nkWhileStmt: genWhile(c, n)
   of nkBlockExpr, nkBlockStmt: genBlock(c, n)

tests/destructor/t23837.nim

@@ -48,4 +48,18 @@ proc main() =
   let s = leakyWrapper()
   echo s
 
-main()
+main()
+
+block:
+  proc foo =
+    var x = "1234"
+    var y = x
+    when nimvm:
+      discard
+    else:
+      var s = x
+      doAssert s == "1234"
+    doAssert y == "1234"
+
+  static: foo()
+  foo()