[AMDGPU] Update test nullptr.ll to use amdgiz environment
[AMDGPU] Update test nullptr.ll to use amdgiz environment

This test needs to be manually updated since it is difficult to do it with script.

Addr space 6 to 23 are only used by r600, therefore only check them for r600.

Differential Revision: https://reviews.llvm.org/D40117


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@319092 91177308-0d34-0410-b5e6-96231b3b80d8

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