Adds a new pass, `splitlarge`, that recursively divides $add/$sub
cells into smaller cells until each cell's width doesn't exceed a
given max_width (128 by default.) An $add/$sub cell's width for
this purpose is defined as the higher of the widths of its two
inputs.
A test was written in Tcl for it, which tests this matrix:
- cell: $add/$sub
- b: unsigned, signed
- a: unsigned, signed
This is the first test for a Silimate pass in Tcl and thus
`run-test.sh` was modified to include it.
Mohamed Gaber