[llvm] [VectorCombine] Precisely calculate the cost in foldInstExtBinop and avoid infinite loop (PR #153069)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM (the CI failure appears to be an unrelated lldb failure)

https://github.com/llvm/llvm-project/pull/153069