Vectorised log() not consistent across different vector sizes

Eigen's vectorised .log() is returning slightly different values depending on the size of the vector since the 3.4.0 release. I believe this is related to the changes made in !315 (merged)

Example on godbolt: https://godbolt.org/z/7froaMq5s

As background, we're experiencing regressions when attempting to upgrade Stan to Eigen 3.4, and found that the tests passed if the changes made in !315 (merged) were reverted.