Implement GCNSubtarget::checkSubtargetFeatures as a canonical place to
check subtarget features for consistency and diagnose any
inconsistencies. To start with, the implementation just checks that
either wavefrontsize32 or wavefrontsize64 is selected.
checkSubtargetFeatures is called at the start of instruction selection.
This is pretty arbitrary. It is just a convenient point at which we have
access to the subtarget that we're going to use for codegenning a
particular function.
