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.nim

@@ -0,0 +1,47 @@
+discard """
+  nimout: '''
+nimvm - when
+nimvm - whenElif
+nimvm - whenElse
+'''
+  output: '''
+when
+whenElif
+whenElse
+'''
+"""
+
+# test both when and when nimvm to ensure proper evaluation
+
+proc compileOrRuntimeProc(s: string) =
+  when nimvm:
+    echo "nimvm - " & s
+  else:
+     echo s
+
+template output(s: string) =
+  static:
+    compileOrRuntimeProc(s)
+  compileOrRuntimeProc(s)
+
+when compiles(1):
+  output("when")
+elif compiles(2):
+  output("fail - whenElif")
+else:
+  output("fail - whenElse")
+
+when compiles(nonexistent):
+  output("fail - when")
+elif compiles(1):
+  output("whenElif")
+else:
+  output("fail - whenElse")
+
+when compiles(nonexistent):
+  output("fail - when")
+elif compiles(nonexistent):
+  output("fail - whenElif")
+else:
+  output("whenElse")
+