[NFC][Sanitizer] Refine the restriction on SizeClassAllocator64::kRegionSize (#86270)

This patch replaces the `SANITIZER_WORDSIZE / 2` with
`sizeof(CompactPtrT) * 8`, replaces hardcoded `4` with
`kCompactPtrScale` in assertion.

GitOrigin-RevId: 4720e3831b814fdd2e8441ee0ac05b6934fdf533
1 file changed