[PATCH] ARM: fix application of de morgan's law

[Tim Northover]

> This would inadvertently enable tail calls when they should not be.

> -  if (!EnableARMTailCalls && !Subtarget->supportsTailCall())
> +  if (!EnableARMTailCalls || !Subtarget->supportsTailCall())

Are you sure this isn't the intended behaviour? It seems to come down
to whether -arm-tail-calls means "do a tail call if you think it's
safe" or "do a tail call even if you think it's unsafe". None of the
surrounding documentation seems clear on the intent, but either
interpretation could be useful for different people.

Tim.