New optimization: --tree-simplify

New optimization has been implemented: --tree-simplify (enabled with
-O1)
It's a small optimization that tries to reduce each expression to a
smaller, faster form. Now it only reduces:
x = x op y;
to
x op= y;
Eg.: "x = x+10;" is changed to "x += 10;"
Also, a few small SSA bugs have been removed.