[tilegx] Revert previous a0 save operation.

  No need to save a0 here, because the code path branch to will not
use a0 anymore, so it's safe to clobber it.
[tilegx] Fix bug in EnsureNotWhite.

  mips code play some tricks, only apply on 32bit processor, on the "length",
we need to adapt those tricks to TileGX.