mem-cache: Set compression bit with its size

mem-cache: Set compression bit with its size

When setting the size of a compressed block, its compressibility
needs to be recalculated based on that, so move such functionality
to be done after the block has been inserted, within setSizeBits.

As a side effect, insertBlock does not need to be overridden
anymore.

Change-Id: I608f876cd2110ac5e394ffad5b29941ba458ba91
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36580
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>