AMDGPU: Add and clarify reserved address spaces (llvm#166486)

Address spaces 10 and 11 are reserved for future use in the sense that
we plain to upstream their use.

Address space 12 is used by LLPC. It is used in a workaround for an
issue with SMEM accesses to PRT buffers that is specific to the LLPC
ecosystem and makes no sense to upstream.

Author: nhaehnle

llvm/docs/AMDGPUUsage.rst

@@ -883,8 +883,9 @@ supported for the ``amdgcn`` target.
      Buffer Fat Pointer                    7               N/A         N/A              160     0
      Buffer Resource                       8               N/A         V#               128     0x00000000000000000000000000000000
      Buffer Strided Pointer (experimental) 9               *TODO*
-     *reserved for downstream use*         10
-     *reserved for downstream use*         11
+     *reserved for future use*             10
+     *reserved for future use*             11
+     *reserved for downstream use (LLPC)*  12
      Streamout Registers                   128             N/A         GS_REGS
      ===================================== =============== =========== ================ ======= ============================