cpu-o3: fix IQ missing mem barriers

cpu-o3: fix IQ missing mem barriers

After commit e2a5063e5f18f902833c84894b0ff103e3371493 some
memory references now tracked as barriers were not having
their completion properly notified to the MemDepUnit.

This patch fixes InstructionQueue and changes MemDepUnit's
completeBarrier to completeInst, which now should be called
for both memory references and barrier instructions.

Change-Id: I28b5f112b45778f6272e71bb3766b364c3d2e7db
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29654
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>