i965/compaction: Simplify jump target code.

i965/compaction: Simplify jump target code.

My attempts to clarify the code with _compacted/_uncompacted prefixed
variables apparently failed. Hopefully this is clearer.

In any case, the previous code wasn't clear enough to gcc to let it
optimize division by a power of two into a shift. No problems now.

Also, the previous code (in the ADD case) didn't work on 32-bit x86, due
to complicated set of interactions best summed up as unsigned division
and compiler optimizations.

Tested-by: Mark Janes <mark.a.janes@intel.com>