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

[Luke Lau via llvm-commits]

lukel97 wrote:

> Only point I'm still a bit hesitant on is if we have some instruction with "weird" passthru usage (i.e. read after VL, etc..).

vslidedowns and vrgathers can read past VL, but we guard against them already via `mayReadPastVL`. 

> Apparently we really don't use anything other than tail undef in practice do we?

It shows up in a lot more places if we stop emitting non-trapping VP intrinsics with EVL tail folding.

We also probably never see tail undef ternary pseudos, because that would mean one of the operands would have to be undef too.

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