This commit changes the allocation policy in Store Set memory
dependence predictor.

Previously we allocate an entry for the load and store instructions
every time when a memory violation is triggered. However, it's not
robust enough and causes many load instructions to be blocked for
issuing.

The current allocation policy only allocates the same entry for the load
and store instructions after both of them have triggered memory
violations before. That is to say, only when a load/store instruction
pair triggers memory violation twice, we do allocate the same entry to
them. This change may cause more memory violation redirections than
before, but it also reduces the number of blocked load instructions.