[NVPTX] Let NVPTX backend detect integer min and max patterns.

Summary:
Let NVPTX backend detect integer min and max patterns during isel and emit intrinsics that enable hardware support.


Reviewers: jholewinski, meheff, jingyue

Subscribers: arsenm, llvm-commits, meheff, jingyue, eliben, jholewinski

Differential Revision: http://reviews.llvm.org/D12377

llvm-svn: 246107