EMERGING MARKETS-Brazil stocks slump to 8-month low as OGX dives

* Batista's OGX tumbles to record low on credit downgrade * Brazil court postpones decision on foreign tax case * Bovespa falls 1.65 pct, Mexico IPC down 0.35 pct By Roberta Vilas Boas SAO PAULO, April 4 (Reuters) - Brazilian stocks slumped to a more than eight-month low on Thursday as billionaire Eike Batista's oil firm OGX tumbled to a record low after a credit downgrade and global iron miner Vale dropped due to uncertainty about a looming tax ruling. Brazil's Bovespa ind