[NVPTX] Fix issue introduced in D10321

Summary:
Really check if %SP is not used in other places, instead of checking only exact
one non-dbg use.

Patched by Xuetian Weng. 

Test Plan:
@foo4 in test/CodeGen/NVPTX/local-stack-frame.ll, create a case that
SP will appear twice.

Reviewers: jholewinski, jingyue

Reviewed By: jingyue

Subscribers: llvm-commits, sfantao, jholewinski

Differential Revision: http://reviews.llvm.org/D10844

llvm-svn: 241099