mem-cache: Fix BDI size calculation

The bitmask field indicates to which base a delta refers, and in
the original paper it is fixed and proportional to the highest
number of bases allowed in the compressed data.

Change-Id: I271bf2e19e0765de52b933eaf6d4fcc2ce25d185
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19748
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

mem-cache: Create BDI Compressor

Implement Base-Delta-Immediate compression, as described in
'Base-Delta-Immediate Compression: Practical Data Compression
for On-Chip Caches'

Change-Id: I7980c340ab53a086b748f4b2108de4adc775fac8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/11412
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>