Optimize compares to powers of 2

Optimize compares to powers of 2
Remove opt_compare and put comparison pass in opt_expr

assuming a [7:0] is unsigned
a >= (1<<x) becomes |a[7:x]
a <  (1<<x) becomes !a[7:x]

Additionally:
a >= 0 becomes constant true,
a < 0 becomes constant false

delete opt_compare.cc
revert opt.cc to commit b7cfb7dbd (remove opt_compare step)