cpu: Failure to restore RAS during squash

During squash of branch predictor history, RAS recovery mess up the
stack because of function "restore" in RAS (src/cpu/pred/ras.cc). In
restore function, it does not update "usedEntries" variable resulting in
restore failure.

To be specific, in order to remove mispredicted call, it uses pop() and
it updates tos. However in order to restore mispredicted ret
instruction, it uses restore() but it does not update tos. This pair of
function call mess up the RAS resulting in many misspeculation.

The solution is to update usedEntries variable as “push” function does.
This is possible because restoration is done with reverse order of push
and pop.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-732

Change-Id: Ia14e71c26d20b2795fd55a6a0dd3284c03570614
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33214
Reviewed-by: Trivikram Reddy <tvreddy@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/pred/ras.cc

@@ -74,4 +74,8 @@ ReturnAddrStack::restore(unsigned top_entry_idx,
     tos = top_entry_idx;
 
     addrStack[tos] = restored;
+
+    if (usedEntries != numEntries) {
+        ++usedEntries;
+    }
 }