Faster and simpler replace in SSB, take two

This is a new version of CL 179343 which had to be reverted.

This problem of the previous CL is that the ComposingSpan that
was part of the replacement text was correctly added during the
replace but was immediately removed because it had a zero-length
size.

Swapping the add and remove blocks solves the problem.

The new non-zero length enforcement also revealed a bug in the
spell checker where we were creating useless range spans.

Change-Id: I59cebd4708af3becc7ab625ae41bc36837f1a1cf