Fix a tblgen subtargetemitter bug, for future Swift support.

This fixes some of the ridiculously complex code for optimizing the
machine model tables that are shared among all processors of a given
target. A9 and Swift both use the "special" feature that maps old
itinerary classes to new machine model defs. They map different
overlapping subsets of instructions, which wasn't handled correctly.

llvm-svn: 183302