DAG: Fix naming crime

Because isOperationCustom was only checking for custom
lowering on illegal types, this was behaving inconsistently
with the other isOperation* functions, so that
isOperationLegalOrCustom != (isOperationLegal || isOperationCustom)

Luckily this is only used in one place which already checks the
type legality on its own.

llvm-svn: 311743