fixes strictdefs with when nimvm (#24409)

ref #24225
related #24306

> Code in branches must not affect semantics of the code that follows
the
`when nimvm` statement. E.g. it must not define symbols that are used in
  the following code.

The test shouldn't have passed when
#24306
would be implemented somehow. Some third packages have already misused
`when nimvm` by defining symbols in the other branch of `when nimvm`.

e.g. in status-im/nim-unittest2#34

```nim
when nimvm:
  discard
else:
  let suiteName {.inject.} = nameParam

use(suiteName)
```

compiler/sempass2.nim

@@ -1301,7 +1301,10 @@ proc track(tracked: PEffects, n: PNode) =
       let last = lastSon(child)
       track(tracked, last)
   of nkCaseStmt: trackCase(tracked, n)
-  of nkWhen, nkIfStmt, nkIfExpr: trackIf(tracked, n)
+  of nkWhen: # This should be a "when nimvm" node.
+    track(tracked, n[0][1])
+    track(tracked, n[1][0])
+  of nkIfStmt, nkIfExpr: trackIf(tracked, n)
   of nkBlockStmt, nkBlockExpr: trackBlock(tracked, n[1])
   of nkWhileStmt:
     # 'while true' loop?

tests/init/tlet.nim

@@ -1,3 +1,7 @@
+discard """
+  joinable: false
+"""
+
 {.experimental: "strictDefs".}
 
 proc bar(x: out string) =
@@ -53,3 +57,29 @@ proc foo() =
 
 static: foo()
 foo()
+
+proc foo2 =
+  when nimvm:
+    discard
+  else:
+    let x = 1
+  doAssert x == 1
+
+  when false:
+    discard
+  else:
+    let y = 2
+
+  doAssert y == 2
+
+  const e = 1
+  when e == 0:
+    discard
+  elif e == 1:
+    let z = 3
+  else:
+    discard
+
+  doAssert z == 3
+
+foo2()