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.

* mdp: implement SSIT with sram

* mdp: use robIdx instead of sqIdx

Dispatch refactor moves lsq enq to dispatch2, as a result, mdp can not
get correct sqIdx in dispatch. Unlike robIdx, it is hard to maintain a
"speculatively assigned" sqIdx, as it is hard to track store insts in
dispatch queue. Yet we can still use "speculatively assigned" robIdx
for memory dependency predictor.

For now, memory dependency predictor uses "speculatively assigned"
robIdx to track inflight store.

However, sqIdx is still used to track those store which's addr is valid
but data it not valid. When load insts try to get forward data from
those store, load insts will get that store's sqIdx and wait in RS.
They will not waken until store data with that sqIdx is issued.

* mdp: add track robIdx recover logic