AMDGPU: Fix crashes on unknown processor name

If the processor name failed to parse for amdgcn,
the resulting output would have R600 ISA in it.

If the processor name was missing or invalid for R600,
the wavefront size would not be set and there would be
crashes from missing itinerary data.

Fixes crashes in future commit caused by dividing by the unset/0
wavefront size.

llvm-svn: 271561