[Inlining] Set inline-deferral-scale to 2.

Summary:
This patch sets inline-deferral-scale to 2.

Both internal and SPEC benchmarking show that 2 is the best number
among -1, 2, 3, and 4.

inline-deferral-scale  SPECint2006
------------------------------------------------------------
                   -1  38.0 (the default without this patch)
                    2  38.5
                    3  38.1
                    4  38.1

With the new default number, shouldBeDeferred returns true if:

  TotalCost < IC.getCost() * 2

where

  TotalCost is TotalSecondaryCost + IC.getCost() * NumCallerUsers.

If TotalCost >= 0 and NumCallerUsers >= 2, then
TotalCost >= IC.getCost() * 2, so shouldBeDeferred returns true only
when NumCallerUsers is 1.

Now, if TotalSecondaryCost < 0, which can happen if
InlineConstants::LastCallToStaticBonus, a huge number, has been
subtracted from TotalSecondaryCost, then TotalCost may be negative.
In this case, shouldBeDeferred may return true even when
NumCallerUsers >= 2.

Reviewers: davidxl, nikic

Reviewed By: davidxl

Subscribers: xbolva00, hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80229