[llvm] [MIR] Introduce split-from VRM state and lr-split instr flag (PR #194494)

[Quentin Colombet via llvm-commits]

qcolombet wrote:

@arsenm I'm exposing the VRM state in this PR as well as the lr-split flag (which split kit is supposed to set on copy instructions inserted by splitting).
One commit for each feature, then third commit uses these features to dramatically reduce the size of the regalloc reproducer introduced in #193880 .

If the approach seems sensible. I'll clean up and do proper PRs

https://github.com/llvm/llvm-project/pull/194494