mem-cache: Implement a perfect compressor

mem-cache: Implement a perfect compressor

Implement a perfect compressor that always manages to compresses data
exactly to its maximum allowed compression ratio. This allows tracking
a compression upper bound.

Change-Id: Ibc68bf2dc84b75207795d5ba6304b9ed6dbeae8f
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21160
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>