IMPORT: eb32/64: optimize insert for modern CPUs

IMPORT: eb32/64: optimize insert for modern CPUs

Similar to previous patches, let's improve the insert() descent loop to
avoid discovering mandatory data too late. The change here is even
simpler than previous ones, a prefetch was installed and troot is
calculated before last instruction in a speculative way. This was enough
to gain +50% insertion rate on random data.

This is ebtree commit e893f8cc4d44b10f406b9d1d78bd4a9bd9183ccf.