[AMDGPU] Assembler: rename amd_kernel_code_t asm names according to spec

Summary:
Also removed duplicate code from AMDGPUTargetAsmStreamer.
This change only change how amd_kernel_code_t is parsed and printed. No variable names are changed.

Reviewers: vpykhtin, tstellarAMD

Subscribers: arsenm, wdng, nhaehnle

Differential Revision: https://reviews.llvm.org/D24296

llvm-svn: 281028