[VPlan] Use wide IV if scalar lanes > 0 are used with scalable vectors. (#169796)
For scalable vectors, VPScsalarIVStepsRecipe cannot create all scalar
step values. At the moment, it creates a vector, in addition to to the
first lane. The only supported case for this is when only the last lane
is used. A recipe should not set both scalar and vector values.
Instead, we can simply use a vector induction. It would also be possible
to preserve the current vector code-gen, by creating VPInstructions
based on the first lane of VPScalarIVStepsRecipe, but using a vector
induction seems simpler.
PR: https://github.com/llvm/llvm-project/pull/169796
llvm-project
25ab47bd - [VPlan] Use wide IV if scalar lanes > 0 are used with scalable vectors. (#169796)
