Add a flag to drcachesim to set cache warmup by loaded fraction

Created by: snehasish

drcachesim currently supports cache warm up by absolute number of references. We would like to support cache warmup determined by what fraction of the cache has been loaded -- eg "the cache is warm once 90% of its blocks have been loaded".

We determine the loaded fraction of the cache hierarchy by enumerating valid blocks in the last level cache only.

Implementation:

1. Add warmup fraction knob to simulator_t
2. Add counter num_valid_blocks to caching_device_t. Count of valid blocks are maintained by incrementing on load miss. This counter will need to be decremented on a) flush b) coherence invalidations (#1726 (closed))
3. Check loaded fraction to determine warmup reached in cache_simulator_t::process_memref. Only use the fraction from llcache as we are simulating a two level hierarchy with a single last level cache (#1715 (closed)).