[NVPTX] Adds a new address space inference pass.

Summary:
The old address space inference pass (NVPTXFavorNonGenericAddrSpaces) is unable
to convert the address space of a pointer induction variable. This patch adds a
new pass called NVPTXInferAddressSpaces that overcomes that limitation using a
fixed-point data-flow analysis (see the file header comments for details).

The new pass is experimental and not enabled by default. Users can turn
it on by setting the -nvptx-use-infer-addrspace flag of llc.

Reviewers: jholewinski, tra, jlebar

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D17965