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

[Luke Lau via llvm-commits]

lukel97 wrote:

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

Off the top of my head, vsadd etc. have extra implicit defs, but I'm not sure if it's safe to change their VL. Fault-first loads have an extra explicit def, but I don't think they're safe to reduce either

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