Re-commit r98683 ("remove redundant writeback flag from ARM address mode 6")

Re-commit r98683 ("remove redundant writeback flag from ARM address mode 6")
with changes to add a separate optional register update argument.  Change all
the NEON instructions with address register writeback to use it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99095 91177308-0d34-0410-b5e6-96231b3b80d8