[NVPTX] Added a feature to use short pointers for const/local/shared AS.

Const/local/shared address spaces are all < 4GB and we can always use
32-bit pointers to access them. This has substantial performance impact
on kernels that uses shared memory for intermediary results.

The feature is disabled by default.

Differential Revision: https://reviews.llvm.org/D46147

llvm-svn: 331941