Fix collapse mask tracking for recombine steps.

The recombine loop for cm was correct if one started at 1 block,
 but was wrong otherwise (for a test case, convert 2 recombined
 blocks back to 4 with an initial cm of 0x3; the result should be
 0xF, but instead you get 0x7).
The recombine loop for fill was always wrong (for a test case,
 combine 8 blocks down to 1 with an initial fill=0xFE; the low bit
 remains unset).
This now properly interleaves and deinterleaves bits for these
 steps, which avoids declaring collapses (and skipping folding)
 where none, in fact, occurred.