Further reorganize pmap_invalidate TLB code.

Split calculation of mask for shootdown IPI and local
invalidation. Reorder IPI before local.

Suggested by: alc
Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
Approved by: re (rgrimes)
Differential revision: https://reviews.freebsd.org/D17277