MAINT: restore auto-vectorization of inplace operations #8852
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GCC 6/7 lost the ability to vectorize inplace operations with our
current hinting. This causes inplace operations to become slower than
out of place operations which is bad, especially as we automatically
avoid temporaries now.
This issue has been filed in GCC PR80198.
Luckily gcc also has a no loop dependence pragma which we can use to
enforce the vectorization in the inplace code path.
In the inplace scalar path an extra code hint is sufficient.