At the moment, the primary induction variable for the vector loop is
created as part of the skeleton creation. This is tied to creating the
vector loop latch outside of VPlan. This prevents from modeling the
*whole* vector loop in VPlan, which in turn is required to model
preheader and exit blocks in VPlan as well.
This patch introduces a new recipe VPCanonicalIVRecipe to represent the
primary IV in VPlan and InductionIncrement{NUW} opcodes for
VPInstruction to model the increment.
This allows us to partly retire createInductionVariable. At the moment,
a bit of patching up is done after executing all blocks in the plan.
This is still WIP as there are a couple of test failures left to fix,
but I'd like to share it so the general direction can be reviewed.
Document \p PrimInd
Rename PrimInd to CanonicalIV or vice-versa rename CanonicalIV to PrimaryIV, throughout?