arch,cpu: Get rid of the IsMemRef StaticInst flag.

arch,cpu: Get rid of the IsMemRef StaticInst flag.

A comment at the top of StaticInstFlags.py says that if IsMemRef is set,
exactly one of IsStore or IsLoad will be set. That's not strictly true
since IsAtomic may be set as well, in which case neither IsStore or
IsLoad will be set (in one example I found).

The isMemRef accessor still exists, and now just ors the IsStore,
IsLoad, and IsAtomic flags.

Change-Id: Ic5ff104da68978273977a6eff2abab5dd0ae7fda
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33744
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>