[llvm] [CodeGen] Preserve IMPLICIT_DEF for subregister joins (PR #218348)

[Karl Hylen via llvm-commits]

Hylen wrote:

> When I was updating tests I noticed that the upstream targets with subregister liveness enabled get worse code generation in many of the tests. Some tests even start spilling after disabling the IMPLICIT_DEF removal. So am I moving in the right direction by disabling the optimisation here? Or do I need to find a more narrow correctness condition and restrict when we retain the IMPLICIT_DEFs?

@XChy , @arsenm , @qcolombet , @sommerlukas: Can any of you please provide feedback on how to proceed with this?

https://github.com/llvm/llvm-project/pull/218348