[llvm] [CodeGen] Introduce MIR-level target-independent rematerialization helper (PR #177080)

[Quentin Colombet via llvm-commits]

https://github.com/qcolombet approved this pull request.

Looks good to me, raised a few points.
I let you correct them, no need for a new review round, unless you want to.

https://github.com/llvm/llvm-project/pull/177080