[NVPTX] Allow undef value as global initializer

Summary:
__shared__ variable may now emit undef value as initializer, do not
throw error on that.

Test Plan: test/CodeGen/NVPTX/global-addrspace.ll

Patch by Xuetian Weng

Reviewers: jholewinski, tra, jingyue

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D12242

llvm-svn: 245785