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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

Do we get any impact if we remove `MI.getNumDefs() != 1` in is candidate? Not sure if that would be related here.

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