[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.
> 
> 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

> Fault-first loads have an extra explicit def, but I don't think they're safe to reduce either

I had played around with a patch in the past that changed this line to `MI.getNumExplicitDefs() != 1` to avoid the fault-first loads. `vsadd` and others who mess with vxsat are not part of `isSupportedInstr`. I think we could probably remove this line. It was placed here to guard against passthru. I think this would give you more optimization.

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