[NVPTX] truncating 64-bit to 32-bit is free

Summary:
Add an LSR test that exercises isTruncateFree. Without this change, LSR creates
another indvar representing the truncated value.

Reviewers: jholewinski, eliben

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D12058

llvm-svn: 245611