[VPlan] Add transformation to narrow interleave groups.

This patch adds a new narrowInterleaveGroups transfrom, which tries
convert a plan with interleave groups with VF elements to a plan that
instead replaces the interleave groups with wide loads and stores
processing VF elements.

This effectively is a very simple form of loop-aware SLP, where we
use interleave groups to identify candidates.

This initial version is quite restricted and hopefully serves as a
starting point for how to best model those kinds of transforms. For now
it only transforms load interleave groups feeding store groups.

Depends on #106431.

This lands the main parts of the approved
https://github.com/llvm/llvm-project/pull/106441 as suggested to break
things up a bit more.

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