mem-cache: Use the compression factor to co-allocate

mem-cache: Use the compression factor to co-allocate

The compression factor of a block is measured according to the maximum
achievable compression ratio.

For example, if up to 4 blocks can co-allocate in a superblock, and
a cache line has 512 bits, the possible compression factors are 1
(uncompressed, <=512 bits), 2 (compressed, <=256 bits), 4 (compressed,
<=128 bits).

This is an approach similar to the one described in "Yet Another
Compressed Cache", by Sardashti et al.

Change-Id: I52ef36989f3eeef6fc8890132a57f995ef9c5258
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36581
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>