[AMDGPU] Add an additional Code Object V3 assembler example
[AMDGPU] Add an additional Code Object V3 assembler example

Document the intended use of the `.amdgcn.next_free_{s,v}gpr` in the
context of multiple kernels and functions.

Differential Revision: https://reviews.llvm.org/D59949

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@357289 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm