[FuncSpec][NFC] Avoid redundant computations of DominatorTree/LoopInfo

The `FunctionSpecialization` pass needs loop analysis results for its
cost function. For this purpose, it computes the `DominatorTree` and
`LoopInfo` for a function in `getSpecializationBonus`.  This function,
however, is called O(number of call sites x number of arguments), but
the DominatorTree/LoopInfo can be computed just once.

This patch plugs into the PassManager infrastructure to obtain
LoopInfo for a function and removes ad-hoc computation from
`getSpecializatioBonus`.

Reviewed By: ChuanqiXu, labrinea

Differential Revision: https://reviews.llvm.org/D136332