[XLA:CPU] Fix test case for vectorized Exp and Tanh to actually vectorize

I just noticed that the test case for ArrayElementwiseOpTest::ExpF32sVector and
possibly for ArrayElementwiseOpTest::ExpF32sVector does not actually vectorize
the intrinsic calls.  This is most likely a very recent regression because I
remember fixing at least one issue in the emitter demonstrated by the test.
Despite that I think the current approach is better since we have unit tests
that check that we at least vectorize the vector-of-F32's case.

PiperOrigin-RevId: 184918373