[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Philip Reames via llvm-commits]

preames wrote:

> I see this for both vmv.s.x and vmv.v.x. It looks like maybe we broke the don't care on tail policy somehow?

I think I see what happened here.  InitUndef is replacing undef/noreg/implicit_def operands with well defined sources, and we're no longer considering them to have a undef passthrough.  

The downstream effect of this is magnified by another pass we have downstream, so not your problem.  But I think this does explain some of the diffs I called out in the other review.  Can we plausible move InitUndef to just after the new placement here?

https://github.com/llvm/llvm-project/pull/91440