[VPlan] Scalarize to first-lane-only directly on VPlan
This is needed to enable subsequent https://github.com/llvm/llvm-project/pull/182595.
I don't think we can fully port all scalarization logic from the legacy
path to VPlan-based right now because that would require us to introduce
interleave groups much earlier in VPlan pipeline, and without that we
can't really `assert` this new decision matches the previous CM-based
one. And without those `assert`s it's really hard to ensure we properly
port all the previous logic.
As such, I decided just to implement something much simpler that would
be enough for #182595. However, we perform this transformation before
delegating to the old CM-based decision, so it **is** effective
immediately and taking precedence even for consecutive loads/stores
right away.
Depends on https://github.com/llvm/llvm-project/pull/182592 but is stacked on
top of https://github.com/llvm/llvm-project/pull/182594 to enable linear
stacking for https://github.com/llvm/llvm-project/pull/182595.
