[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 approved this pull request.

LGTM

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..).  I can't think of any, and spot checking existing code doesn't seem to reveal in any special casing, so I think we're okay here.  Both passthru for TU/MU and FMA style extra sources should be fine with the code as written. 

I guess one other point is that I'm surprised this has so little impact on the tests.  Apparently we really don't use anything other than tail undef in practice do we?

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