[VPlan] Keep start value of VPWidenPHIRecipe as VPValue.

[fhahn]

Summary

Similar to D92129, update VPWidenPHIRecipe to manage the start value as
VPValue. This allows adjusting the start value as a VPlan transform,
which will be used in a follow-up patch to support reductions during
epilogue vectorization.

[fhahn]

Update after renaming to getLiveInIRValue.

Also updated documentation for VPWidenPHIRecipe to describe when the start value is managed as first operand.

[gilr]

I wonder if the code from fixReductions() could be moved (almost as-is) to widenPHIInstruction() as a first step?

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

4595

RK seems to be the only use of RdxDesc. Can be extracted in tryToCreateWidenRecipe() and stored as an ingredient in the recipe.

4599

Intentionally assigning StartV to itself? (symmetry with the else clause?)

[fhahn]

In D93975#2480166, @gilr wrote:

I wonder if the code from fixReductions() could be moved (almost as-is) to widenPHIInstruction() as a first step?

Thanks for the suggestion, I put up D94175 to move the code and keep RdxDesc in the recipe as a first step. The current patch is much simpler now.

[fhahn]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

4599

It's not needed. (can. be Iden = StartV;). Applied to D94175.

[gilr]

LGTM!

[fhahn]

rebase after parent patch landed.