interp: fix several bugs related to constant vs dirty values

* Loading from a dirty global must be done at runtime (!). For some
  reason this wasn't already the case.
* Global variables somehow had IsConstant() the wrong way round,
  returning the inverse from what they should.
* Do binary and logical operations at runtime if necessary, relying on
  const propagation in the IR builder.
* Don't try to interpret functions that take a dirty parameter. Call
  them at runtime.

interp/values.go

@@ -127,9 +127,9 @@ func (v *GlobalValue) Type() llvm.Type {
 // IsConstant returns true if this global is not dirty, false otherwise.
 func (v *GlobalValue) IsConstant() bool {
 	if _, ok := v.Eval.dirtyGlobals[v.Underlying]; ok {
-		return true
+		return false
 	}
-	return false
+	return true
 }
 
 // Load returns the initializer of the global variable.