amd64: micro-optimize vptopte()/vtopde() further

Eliminate shlq $3,address shift after masking of the va is done, which
is needed to convert pt_entry_t[] array index into byte offset.
Do it by preshifting the mask, and compensating the right shift of va.

Suggested by: alc
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33786