mem-cache: Use shouldAllocate() instead of CPack's decompress()

mem-cache: Use shouldAllocate() instead of CPack's decompress()

Split decompression functionality using the proper function to
determine if a dictionary entry should be allocated after
decompression or not.

Change-Id: I4995304f4c4508c03c9fc1685f04511622969556
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21146
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>