when statements branches exit early (#17143)

When statement branches exit early outside of nimvm. In nimvm it seems that all
sides of the branches must be evaluated as the code gen happens at a later
stage, this remains intact.

tests/whenstmt/twhen_macro.nim

@@ -0,0 +1,24 @@
+import macros
+
+discard """
+  output: '''
+when - test
+'''
+"""
+
+# test that when stmt works from within a macro
+
+macro output(s: string, xs: varargs[untyped]): auto =
+  result = quote do:
+    when compiles(`s`):
+      "when - " & `s`
+    elif compiles(`s`):
+      "elif - " & `s`
+      # should never get here so this should not break
+      broken.xs
+    else:
+      "else - " & `s`
+      # should never get here so this should not break
+      more.broken.xs
+
+echo output("test")