[llvm] [CodeGen] Refactor `determineCalleeSaves`. (PR #166763)

[Sam Elliott via llvm-commits]

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lenary wrote:

Please can you show a more comprehensive test, so we can see the changes from saving CSRs early vs not?

For instance, how does saving a callee-save unconditionally (and early?) affect functions which make a call? Or those that both make a call and need to use the callee-saved register.

https://github.com/llvm/llvm-project/pull/166763