AMDGPU: bump AS.MAX_COMMON_ADDRESS to 6 since 32-bit addr space

32-bit constant address space is declared as 6, so the
maximum number of address spaces is 6, not 5.

Fixes "LLVM ERROR: Pointer address space out of range".

v5: rename MAX_COMMON_ADDRESS to MAX_AMDGPU_ADDRESS
v4: - fix compilation issues
    - fix out of bounds access
v3: use static_assert()
v2: add a very simple test for 32-bit addr space

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106630
llvm-svn: 340417