cpu-o3: Prevent a mistarget from sending execution on an incorrect path
cpu-o3: Prevent a mistarget from sending execution on an incorrect path

This fixes the unlikely but possible following case :

- Assume cond/uncond direct branch A jumping to next branch (PC + 4 in ARM). From
the point of view of the PCState object, the instruction is not branching
(PCState::branching() will return false since it tests whether nextPC != PC + 4 for ARM).
This gets cached in the BTB.

- Assume another cond branch B that is predicted taken but uses the PCState object of the
first branch A from the BTB due to a partial tag match (BTB is not fully tagged).

- At decode, the mistarget will be detected because the target given by the BTB does
not match the target encoded in the instruction B. However, to determine what PC to send to
fetch, decode looks at inst->pcState().branching(), which returns false because the PCState
object has PC X, and nextPC X + 4 (ARM case). Therefore, Decode sends the
fallthrough address of branch B, despite it being predicted taken. If the prediction is
correct, Exec will not realize that the target is wrong since it is the Decode stage's job.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-947

Change-Id: Ia3b960bb660bdfd3c348988d6532735fa3268990
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/46260
Reviewed-by: Nathanael Premillieu <e171d5cbe239782f7aa1d428343f01f9a7e19e5f@huawei.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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