SSE4 / 16x8bit target

The main new thing in this branch is a new SSE4 target optimized for computation on 8-bit datatypes.  So the mask is 8-bits instead of 32, programCount == 16 for it, etc.  Code generated for this target looks quite good in general.

Beyond writing the target backend file, there were a lot of places in the system that assumed masks would only either have 1 or 32 bit elements that are fixed in some of the checkins.