AMDGPU: Remove global isGCN predicates

These are problematic because they apply to everything,
and can easily clobber whatever more specific predicate
you are trying to add to a function.

Currently instructions use SubtargetPredicate/PredicateControl
to apply this to patterns applied to an instruction definition,
but not to free standing Pats. Add a wrapper around Pat
so the special PredicateControls requirements can be appended
to the final predicate list like how Mips does it.

llvm-svn: 314742