[NVPTX] Set NVPTXTTI::getInliningThresholdMultiplier to 5.

Summary:
Calls on NVPTX are unusually expensive (for one thing, lots of state
needs to be saved to memory, which is slow), so make the inlininer much
more aggressive.

Reviewers: chandlerc

Subscribers: jholewinski, llvm-commits, tra

Differential Revision: http://reviews.llvm.org/D18561

llvm-svn: 266406