Add preliminary implementation for GPGPU code generation.

Translate the selected parallel loop body into a ptx string and run it
with cuda driver API. We limit this preliminary implementation to
target the following special test cases:
  - Support only 2-dimensional parallel loops with or without only one
    innermost non-parallel loop.
  - Support write memory access to only one array in a SCoP.

Contributed by:  Yabin Hu <yabin.hwu@gmail.com>

llvm-svn: 160164