[TTI] Add getInliningThresholdMultiplier.

[TTI] Add getInliningThresholdMultiplier.

Summary:
InlineCost's threshold is multiplied by this value.  This lets us adjust
the inlining threshold up or down on a per-target basis.  For example,
we might want to increase the threshold on targets where calls are
unusually expensive.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18560

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266405 91177308-0d34-0410-b5e6-96231b3b80d8