Reorder add clobbers
Mini-pass to reconstruct absolute-indexed addresses.

Pseudos that have been moved by (subreg) parts are looked through in
order to utilise absolute-indexed addressing modes more effectively.
Use define_subst to remove some code duplication, carry tweaks.

Don't set carry on the last byte of multi-byte add/sub sequences.
Add -m[no-]symbol-prop flag to make new pass optional.
Fix some warnings, remove some dead code.