pmap: Micro-optimize pmap_remove_pages() on amd64 and arm64

Reduce the live ranges for three variables so that they do not span the
call to PHYS_TO_VM_PAGE(). This enables the compiler to generate
slightly smaller machine code.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31161