interp: fix sync/atomic.Value load/store methods

These methods do some unsafe pointer casting but can be assumed to not
have significant side effects. Simply call these functions at runtime
instead of compile time.

This is a partial fix for importing image/png.

interp/scan.go

@@ -62,6 +62,10 @@ func (e *evalPackage) hasSideEffects(fn llvm.Value) (*sideEffectResult, *Error)
 		return &sideEffectResult{severity: sideEffectNone}, nil
 	case name == "llvm.dbg.value":
 		return &sideEffectResult{severity: sideEffectNone}, nil
+	case name == "(*sync/atomic.Value).Load" || name == "(*sync/atomic.Value).Store":
+		// These functions do some unsafe pointer loading/storing but are
+		// otherwise safe.
+		return &sideEffectResult{severity: sideEffectLimited}, nil
 	case strings.HasPrefix(name, "llvm.lifetime."):
 		return &sideEffectResult{severity: sideEffectNone}, nil
 	}