[AMDGPU] Fix +DumpCode to print an entry label for the first function
The +DumpCode attribute is a horrible hack in AMDGPU to embed the
disassembly of the generated code into the elf file. It is used by LLPC
to implement an extension that allows the application to read back the
disassembly of the code.
It tries to print an entry label at the start of every function, but
that didn't work for the first function in the module because
DumpCodeInstEmitter wasn't initialised until EmitFunctionBodyStart
which is too late.
Reviewers: arsenm, tpr, kzhuravl
Reviewed By: arsenm
Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits
Differential Revision: https://reviews.llvm.org/D63712