[NFC][VPlan] Split `makeMemOpWideningDecisions` into subpasses
The idea is to have handling of strided memory operations (either from
https://github.com/llvm/llvm-project/pull/147297 or for VPlan-based
multiversioning for unit-strided accesses) done after some mandatory
processing has been performed (e.g., some types **must** be scalarized)
but before legacy CM's decision to widen (gather/scatter) or scalarize
has been committed.
And in longer term, we can uplift all other memory widening decision to
be done here directly at VPlan level. I expect this structure would also
be beneficial for that.
llvm-project
c5dff262 - [NFC][VPlan] Split `makeMemOpWideningDecisions` into subpasses
