GlobalISel: Fix some artifact combiner worklist inconsistencies

In one case, UpdateDefs was not getting set and a dead SmallVector
constructed. In another, it was adding new vreg defs to the updated
set which should be unnecessary. This also wasn't considering the
multiple defs of G_UNMERGE_VALUES.

Also increase the small vector sizes for merge/unmerge operands to the
usual semi-arbitrary 8. While debugging these, I'm usually seeing
merges and unmerges with at least 4 uses/defs.

I haven't run into an actual problem from any of these though.