Check there are no side effects before optimizing away compile time expressions. (#9934)

tests/vm/tcompiletimesideeffects.nim

@@ -0,0 +1,42 @@
+discard """
+  output:
+'''
+@[0, 1, 2]
+@[3, 4, 5]
+@[0, 1, 2]
+3
+4
+'''
+"""
+
+template runNTimes(n: int, f : untyped) : untyped =
+  var accum: seq[type(f)]
+  for i in 0..n-1:
+    accum.add(f)
+  accum
+
+var state {.compileTime.} : int = 0
+proc fill(): int {.compileTime.} =
+  result = state
+  inc state
+
+# invoke fill() at compile time as a compile time expression
+const C1 = runNTimes(3, fill())
+echo C1
+
+# invoke fill() at compile time as a set of compile time statements
+const C2 =
+  block:
+    runNTimes(3, fill())
+echo C2
+
+# invoke fill() at compile time after a compile time reset of state
+const C3 =
+  block:
+    state = 0
+    runNTimes(3, fill())
+echo C3
+
+# evaluate fill() at compile time and use the results at runtime
+echo fill()
+echo fill()