Add an alternative to test VPlan in more isolation via a new
`vplan-test-transform` option, which builds VPlan0 for each loop in the
input IR and then can invoke a set of transforms on it.
In order to allow different recipe types to be created, a new
widen-from-metadata transform is added, which transforms VPInstructions
to different recipes, based on custom !vplan.widen metadata. Currently
this supports creating widen & replicate recipes, but can easily be
extended in the future.
Currently the handling is intentionally bare-bones, to be extended
gradually as needed.
PR: https://github.com/llvm/llvm-project/pull/178522
3df1c6f88b