[VPlan] Add prepareForExecution to set up live-ins (NFC).

[fhahn]

Summary

This patch adds a new prepareForExecution helper to set up live-ins, so
VPTransformState doesn't need to hold values like TripCount.

This also requires making the trip count operand for ActiveLaneMask
explicit in VPlan.

[Ayal]

This looks good to me, adding some optional suggestions.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

7965

Induction/CanonicalIV is another live-in that could be handled by prepareForExecution(), but soon D113223 will replace it and pass CanonicalIVStartValue to prepareForExecution() instead?

7970

Best placed right before calling BestVPlan.execute()?

Can alternatively have execute() call (a protected) prepareForExecution(), but in any case the caller needs to pass live-in Values separately from State.

Can alternatively be called prepareToExecute().

8463–8464

Now that the comment is removed, so can the {} be removed...
Or set VPValue *TC = Plan->getOrCreateTripCount() analogous to the ICmpULE else case below.

llvm/lib/Transforms/Vectorize/VPlan.cpp

815

Slightly more logical to first handle trip count and then backedge taken count?

llvm/lib/Transforms/Vectorize/VPlan.h

2144

Perhaps TripCount should appear first, before BackedgeTakenCount. In any case, may be good to mention how the two relate.

2178

Can alternatively add TripCount et al. to VPValuesToFree when constructed, placing the remember-to-deallocate next to the allocation.

2216

Slightly more logical to list TripCount first and then BackedgeTakenCount?

[fhahn]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

7965

Yes, CanonicalIV won't be needed after D113223 and I'll updated D113223 to use prepareToExecute to handle CanonicalIVStartValue

7970

moved and renamed in the committed version

8463–8464

updated to use TC variable

llvm/lib/Transforms/Vectorize/VPlan.cpp

815

reordered

llvm/lib/Transforms/Vectorize/VPlan.h

2144

mentioned that BTC is TripCount -1 in the comment for BTC.

2216

reordered