[llvm] [CodeGen][MISched] Handle empty sized resource usage. (PR #75951)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> It seems to me that, if a resource is required to be available, it means that the instruction needs to use it? Why not just mark it for at least 1 cycle usage?

Hi @fpetrogalli I have just run spec2006 comparing scheduler that uses `ReleaseAtCycle = [0]` scheduler that uses `sed ReleaseAtCycle = [0]/ReleaseAtCycle = [1]` on one of our downstream scheduler model. Unfortunately I am seeing significant regressions on spec2006 with the change. I am also skeptical about making this chance since it will force all other scheduler models that use this to retune their model.

As a result, I am in favor of this patch instead of a patch that imposes a `ReleaseAtCycle > 0` requirement. WDYT?

https://github.com/llvm/llvm-project/pull/75951