cpu: MinorCPU handling IsSquashAfter flag

cpu: MinorCPU handling IsSquashAfter flag

MinorCPU was not handling IsSquashAfter flagged instructions. The
behaviour was to force a branch (hence enforcing refetching) for
SerializeAfter instructions only. This has now been extended to
SquashAfter in order to correctly support ISB barrier instruction
behaviour.

Change-Id: Ie525b23350b0de121372d3b98b433e36b097d5c4
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5702
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>