[ARM] Implement isLoad/StoreFromStackSlot for MVE stack stores accesses

This implements the isLoadFromStackSlot and isStoreToStackSlot for MVE
MVE_VSTRWU32 and MVE_VLDRWU32 functions. They behave the same as many
other loads/stores, expecting a FI in Op1 and zero offset in Op2. At the
same time this alters VLDR_P0_off and VSTR_P0_off to use the same code
too, as they too should be returning VPR in Op0, take a FI in Op1 and
zero offset in Op2.

Differential Revision: https://reviews.llvm.org/D106797