[ModuleInliner] Add MLPriority
This patch adds MLPriority as the first step toward the ML-based
function inlining with the module inliner.
For now, MLPriority is completely identical to CostPriority.
Once this patch lands, I'm planning to:
- integrate NoInferenceModelRunner,
- memoize the priority computation so that the priority remains the
same for given values of metrics even with the noise injected during
training, and
- port/take more features into account.
Differential Revision: https://reviews.llvm.org/D139140