[llvm] [SLP]Fix graph traversal in getSpillCost (PR #124984)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Hm, I may have noticed one other thing.  I think you want to replace "&*PrevInstIt != PrevInst" with !isVectorized(&*PrevInstIt) in the NumCall++ condition.  It doesn't seem to cause a measurable test diff in tree, but I think the current code could over-count calls which are all part of one vector bundle.  

This change applied on it's own to the prior version of the code causes the same changes in the tests as this review.  So it's not purely a nop.  

https://github.com/llvm/llvm-project/pull/124984