AMDGPU/SI: Emit amd_kernel_code_t in EmitFunctionBodyStart()

Summary:
This way the function symbol points to the start of amd_kernel_code_t
rather than the start of the function.

Reviewers: arsenm

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10705

llvm-svn: 240829