[NVPTX] Add an NVPTX-specific TargetTransformInfo

Summary:
It currently only implements hasBranchDivergence, and will be extended
in later diffs.

Split from D6188.

Test Plan: make check-all

Reviewers: jholewinski

Reviewed By: jholewinski

Subscribers: llvm-commits, meheff, eliben, jholewinski

Differential Revision: http://reviews.llvm.org/D6195