[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

llvm-svn: 266405