De-nest if's and fix mix-up

Two deeply nested if's obscured that the sense of the conditions was
mixed up. Amazingly, TableGen's output is exactly the same even with the
sense of the tests fixed; it seems that all of TableGen's conversions
are symmetric so that the inverted sense was nonetheless correct "by
accident". As such, I couldn't come up with a test case.

If there does in fact exist a non-symmetric conversion in TableGen's
type system, then a test case should be prepared.

Despite the symmetry, both if's are left in place for robustness in the
face of future changes.

Review by Jakob.

llvm-svn: 164195