[llvm] [MachineScheduler] Remove TODO and replace computation with variable (PR #119551)

[via llvm-commits]

ywgrit wrote:

> Can you please update the PR description to say why the TODO can be dropped? Was it implemented in this commit? [3d59437](https://github.com/llvm/llvm-project/commit/3d594370933b515234c208a85a1c091d3e38d7f7)

3d594370933b515234c208a85a1c091d3e38d7f7 implemented by Javed Absar checks whether the SU must be issued alone, i.e., instruction that must be start/end a group

https://github.com/llvm/llvm-project/pull/119551