Recommit: [FuncSpec][NFC] Refactor finding specialisation opportunities

[recommitting after recommitting a dependency]

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

Change-Id: I7d21081a2479cbdb62deac15f903d6da4f3b8529