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

(cherry picked from commit d411b285bc293a37e062d8fb15b85212ce16abab)