Hide scratch bounds checks behind LLVM_HAVE_BRANCH_AMD_GFX.
Hide scratch bounds checks behind LLVM_HAVE_BRANCH_AMD_GFX.

Scratch bounds checks are currently only supported in amd-gfx LLVM.

googlestadia/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc