Change the names of the TableGen features to match the names used by
AMDGPUSubtarget. "Addressable" refers to the amount that can be accessed
by a single workgroup. Add some explanatory comments. NFC.
The generic subtarget has neither of these features. Rather than forcing
HasMovrel on, it is simpler to expand dynamic vector indexing to a
sequence of compare/select instructions.
NFC for real subtargets.
