[Inlining] Teach shouldBeDeferred to take the total cost into account

Summary:
This patch teaches shouldBeDeferred to take into account the total
cost of inlining.

Suppose we have a call hierarchy {A1,A2,A3,...}->B->C.  (Each of A1,
A2, A3, ... calls B, which in turn calls C.)

Without this patch, shouldBeDeferred essentially returns true if

  TotalSecondaryCost < IC.getCost()

where TotalSecondaryCost is the total cost of inlining B into As.
This means that if B is a small wraper function, for example, it would
get inlined into all of As.  In turn, C gets inlined into all of As.
In other words, shouldBeDeferred ignores the cost of inlining C into
each of As.

This patch adds an option, inline-deferral-scale, to replace the
expression above with:

  TotalCost < Allowance

where

- TotalCost is TotalSecondaryCost + IC.getCost() * # of As, and
- Allowance is IC.getCost() * Scale

For now, the new option defaults to -1, disabling the new scheme.

Reviewers: davidxl

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79138