A good portion of this patch is the extra functions that needed to be implemented to support the test case. e.g. storeRegToStackSlot, loadRegFromStackSlot, eliminateFrameIndex.
Setting ISD::BR_CC to Expand may appear non-obvious on an architecture with branch+cmp instructions. However, I found it much easier to deal with matching the expanded form.
I had to change simm13_lsb0 and simm21_lsb0 to use the Operand<OtherVT> class rather than Operand<i32> in order to keep tablegen happy. This isn't a big deal, but it does seem a shame to lose the uniformity across immediate types when there's not an obvious benefit (I'm hoping a tablegen expert will educate me on what I'm missing here!).