[FuncSpec][NFC] Refactor finding specialisation opportunities

This patch reorders the traversal of function call sites and function
formal parameters to:

* do various argument feasibility checks (`isArgumentInteresting` ) only once per argument, i.e. doing N-args checks instead of N-calls x N-args checks.

* do hash table lookups only once per call site, i.e. N-calls lookups/inserts instead of N-call x N-args lookups/inserts.

Reviewed By: ChuanqiXu, labrinea

Differential Revision: https://reviews.llvm.org/D135968