[NVPTX] Move NVPTXPeephole after NVPTXPrologEpilogPass

Summary:
Offset of frame index is calculated by NVPTXPrologEpilogPass. Before
that the correct offset of stack objects cannot be obtained, which
leads to wrong offset if there are more than 2 frame objects. This patch
move NVPTXPeephole after NVPTXPrologEpilogPass. Because the frame index
is already replaced by %VRFrame in NVPTXPrologEpilogPass, we check
VRFrame register instead, and try to remove the VRFrame if there
is no usage after NVPTXPeephole pass.

Patched by Xuetian Weng. 

Test Plan:
Strengthened test/CodeGen/NVPTX/local-stack-frame.ll to check the
offset calculation based on SP and SPL.

Reviewers: jholewinski, jingyue

Reviewed By: jingyue

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D10853

llvm-svn: 241185