[ARM][MVE] Enable tail predication for loops containing MVE gather/scatters

Widen the scope of memory operations that are allowed to be tail predicated
to include gathers and scatters, such that loops that are auto-vectorized
with the option -enable-arm-maskedgatscat (and actually end up containing
an MVE gather or scatter) can be tail predicated.

Differential Revision: https://reviews.llvm.org/D85138