interp: fix scanning declarations

Declarations would enter an infinite loop when trying to loop over basic
blocks. That was probably an undefined operation, but still somehow
didn't crash the compiler.

Make sure that scanning declarations works as expected.

interp/scan.go

@@ -57,6 +57,9 @@ func (e *Eval) hasSideEffects(fn llvm.Value) *sideEffectResult {
 	case "llvm.dbg.value":
 		return &sideEffectResult{severity: sideEffectNone}
 	}
+	if fn.IsDeclaration() {
+		return &sideEffectResult{severity: sideEffectLimited}
+	}
 	if e.sideEffectFuncs == nil {
 		e.sideEffectFuncs = make(map[llvm.Value]*sideEffectResult)
 	}

interp/scan_test.go

@@ -21,6 +21,7 @@ var scanTestTable = []struct {
 	{"readCleanGlobal", sideEffectNone, []string{"cleanGlobalInt"}},
 	{"readDirtyGlobal", sideEffectLimited, []string{"dirtyGlobalInt"}},
 	{"callFunctionPointer", sideEffectAll, []string{"functionPointer"}},
+	{"getDirtyPointer", sideEffectLimited, nil},
 }
 
 func TestScan(t *testing.T) {

interp/testdata/scan.ll

@@ -45,6 +45,8 @@ define i64 @readDirtyGlobal() {
   ret i64 %global
 }
 
+declare i64* @getDirtyPointer()
+
 @functionPointer = global i64()* null
 define i64 @callFunctionPointer() {
   %fp = load i64()*, i64()** @functionPointer