Extend initial support for primitive types in PTX backend

- Allow i16, i32, i64, float, and double types, using the native .u16,
  .u32, .u64, .f32, and .f64 PTX types.
- Allow loading/storing of all primitive types.
- Allow primitive types to be passed as parameters.
- Allow selection of PTX Version and Shader Model as sub-target attributes.
- Merge integer/floating-point test cases for load/store.
- Use .u32 instead of .s32 to conform to output from NVidia nvcc compiler.

Patch by Justin Holewinski

llvm-svn: 126824