[ARM][Thumb2] Refresh UXTB16 tests to match optimized IR from instcombine

As discussed on D77804, instcombine will have already performed a similar SimplifyMultipleUseDemandedBits call which will break the UXTB16 pattern that was being match in these DAG tests

I've updated the existing tests so that it match the instcombine IR (with a suitable FIXME) and added an equivalent test pattern suggested by @dmgreen