[SLP] Initial vectorization of non-power-of-2 ops. (#77790)

This patch enables vectorization for non-power-of-2 VFs. Initially only
VFs where adding 1 makes the VF a power-of-2, i.e. we can still make
relatively effective use of the vectors.

It relies on the existing target cost-models to return accurate costs
for
non-power-of-2 vectors. I checked mostly AArch64 and X86 and
there the costs seem reasonable for the costs I checked, although
I expect there will be a need to refine both the cost-models and
lowering
to make most effective use of non-power-of-2 SLP vectorization.

Note that re-ordering and shuffling is not implemented for nodes
requiring padding yet to keep the initial implementation simpler.

The feature is guarded by a new flag, off by defaul for now.

PR: https://github.com/llvm/llvm-project/pull/77790