[AMDGPU] Runtime metadata fixes:

  - Verify that runtime metadata is actually valid runtime metadata when assembling, otherwise we could accept the following when assembling, but ocl runtime will reject it:
    .amdgpu_runtime_metadata
    { amd.MDVersion: [ 2, 1 ], amd.RandomUnknownKey, amd.IsaInfo: ...
  - Make IsaInfo optional, and always emit it.

Differential Revision: https://reviews.llvm.org/D30349

llvm-svn: 296324