powerpc64/pmap: Simplify the code path for moea64_pte_replace_native()

Summary:
MOEA64_PTE_REPLACE() is called often with the pmap lock held, and
sometimes with the page pv lock held. The less work done while holding
a lock, the better. Since we are intending to replace the same PTE
(same hash index), we don't need to recalculate anything, just flat
replace the PTE. This cuts more than 200 instructions off the
invalidating code path. In addition, we don't need to replace a PTE
that's not occupied by this PVO.

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D21515