[SLP][NFC] Fix uninitialized ReductionRoot in getTreeCost
ReductionRoot was initialized to nullptr instead of the RdxRoot
parameter. This caused two ScaleCost calls (for MinBWs cast cost and
ReductionBitWidth resize cost) to pass nullptr as the user instruction,
and suppressed the "Reduction Cost" line in debug output. In practice
the scale factor is the same because the tree root's main op and the
reduction root share the same basic block, so this is NFC.
Reviewers:
Pull Request: https://github.com/llvm/llvm-project/pull/189994
