[VPlan] Keep start value in VPWidenIntOrFpInductionRecipe (NFC).

[fhahn]

Summary

This patch updates VPWidenIntOrFpInductionRecipe to hold the start value
for the induction variable. This makes the start value explicit and
allows for adjusting the start value for a VPlan.

The flexibility will be used in further patches.

[fhahn]

Ping :)

rebased and updated to manage start value as VPValue operand

[fhahn]

updated to use a VPValues to manage the start value. Ping :)

[Ayal]

Good step towards representing more Values, and in particular recording InductionDescriptor information, in VPlan!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

8718–8719

getStateValue() can be asserted to be a live-in, i.e., have a null Def, and then we can obtain its underlying Value (the latter possibly asserted to be non null)?

[fhahn]

Update after renaming to getLiveInIRValue.

[fhahn]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

8718–8719

I updated D92281 to include such an assertion in getLiveInIRValue. There will be multiple users, so I think it would make sense to keep the simple helper, but I can also remove it if you think that's preferable.

[Ayal]

This looks good to me, thanks!

[fhahn]

rebased