C_PPAUTO was matching offsets that is a multiple 8. But this
condition is dropped in CL 55610, causing unaligned offset
between 256 and 504 mistakenly matched to some classes, e.g.
C_UAUTO8K. This CL restores this condition, also fixes an
error that C_PPAUTO shouldn't match C_PSAUTO, because the
latter is not guaranteed to be multiple of 8. C_PPAUTO_8 is
unnecessary, removed.

Fixes #21992.

Change-Id: I75d5a0e5f5dc3dae335721fbec1bbcd4a3b862f2
Reviewed-on: https://go-review.googlesource.com/65730Reviewed-by: David Chase <drchase@google.com>