This patch updates the mask creation code to always create compares of
the form (ICMP_ULE, wide canonical IV, backedge-taken-count) up front
when tail folding and introduce active-lane-mask as later
transformation.
This effectively makes (ICMP_ULE, wide canonical IV, backedge-taken-count)
the canonical form for tail-folding early on. Introducing more specific
active-lane-mask recipes is treated as a VPlan-to-VPlan optimization.
This has the advantage of keeping the logic (and complexity) of
introducing active-lane-mask recipes in a single place, instead of
spreading the logic out across multiple functions. It also simplifies
initial VPlan construction and enables treating introducing EVL as
similar optimization.
nit: these “create op”’s should arguably return an op/recipe/vpinstruction rather than a vpvalue, unless they’re expected to potentially fold constants?