[llvm] [SandboxVec] Simple Instruction Interval class (PR #108882)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Same comments as for the dependency graph:
* could use some doxygen
* to me it reads like an analysis and could be placed in https://github.com/llvm/llvm-project/tree/main/llvm/include/llvm/SandboxIR

https://github.com/llvm/llvm-project/pull/108882