[DAG] canCreateUndefOrPoison/isGuaranteedNotToBeUndefOrPoison - ISD::SCALAR_TO_VECTOR assumes undefs in upper elements instead of poison

The ISD::SCALAR_TO_VECTOR node description now assumes the upper elements are poison, not undef.
https://github.com/llvm/llvm-project/blob/39f21429b714d39d9605f041e629f2de61f93053/llvm/include/llvm/CodeGen/ISDOpcodes.h#L661-L667

But canCreateUndefOrPoison still assumes its undefs:
https://github.com/llvm/llvm-project/blob/77229133dabed0b927748f2a5ae1442b0e97f686/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L6099-L6101

As does isGuaranteedNotToBeUndefOrPoison:
https://github.com/llvm/llvm-project/blob/77229133dabed0b927748f2a5ae1442b0e97f686/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L5821-L5829

There might be other ISD::SCALAR_TO_VECTOR code doing something similar (there's a lot of comments that still refer to undef).

Start with llvm/include/llvm/CodeGen/ISDOpcodes.h and the cited canCreateUndefOrPoison and isGuaranteedNotToBeUndefOrPoison sections in llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp. Search for other ISD::SCALAR_TO_VECTOR handling and comments that still describe upper elements as undef. Done means the affected assumptions consistently match the node description's poison semantics.

Written by the indexing model from the issue text.