arch-arm: Unify VLdmStm behaviour when reg out of index

arch-arm: Unify VLdmStm behaviour when reg out of index

The generic VLdmStm class (modelling A32 VLDM/VSTM) is handling a wrong
register list in a inconsistent way. Some instructions are opting
for being decoded as Unknown, while others handle it inside the
macro instruction constructor by manually adjusting the reglist.

Those are two valid implementation of the CONSTRAINT UNPREDICTABLE
behaviour (1 and 3):

"If regs > 16 || (d+regs) > 32 , then one of the following behaviors must
occur:
1) The instruction is UNDEFINED .
2) The instruction executes as NOP .
3)  One or more of the SIMD and floating-point registers are UNKNOWN . If
the instruction specifies writeback, the base register becomes UNKNOWN .
This behavior does not affect any general-purpose registers."

This patch unfies the behaviour by always opting for option 1) over 3)

Change-Id: I4f98409243d5a2ec64113fe9c87e961a391abe94
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23106
Tested-by: kokoro <noreply+kokoro@google.com>