[llvm] [RISCV][VLOPT] Allow users that are passthrus if tail elements aren't demanded (PR #124066)

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

Hold on, this is now an unbounded recursion.  This can cause both stack explosion, and more likely, significant compile time regressions.  

I think you can achieve the same thing by adding a transform which uses the result of checkUsers to clear a pass thru to NoRegister.  (That is, convert an instruction from TA/TU to tail undefined.)  When we visit that instruction, it would propagate up.  

https://github.com/llvm/llvm-project/pull/124066