Reduce memory usage of ComputeEditDistance() by (almost) 50%

ComputeEditDistance() currently keeps two rows of the edit distance matrix in
memory.  That's unnecessary, one row plus one additional element are sufficient.
With this change, strings up to 64 chars can be processed without going to the
heap, compared to 32 chars previously.  (But the main motivation is that the
code gets a bit simpler.)

No intended behavior change.

llvm-svn: 242069