Issue Description:

Protocol methods are modeled as functions generic on Self, so IRGen tries to get the type metadata to satisfy the formal type parameter, even though it's not used for an ObjC or C protocol method:

define void @invoke_protocol_methods(%objc_object*) #​0 {
entry:
  %.Type = call %swift.type* @swift_getObjectType(%objc_object* %0) #​2
  %1 = bitcast %objc_object* %0 to i8*
  call void @mutateSomeState(i8* %1)
  ret void
}

The call is marked readnone so gets optimized away, but we shouldn't emit it at all for the sake of unoptimized code.