iropt: firm does not alloow widening mul operations anymore
only transform x*2.0 => x+x in ARCH_DEP phase to avoid endless recursion with x+x=>2*x rule
iropt: factor out rules to negate values and use them to improve -x*y => x*-y
merge value_not_zero() into value_not_null(), cleanup and move it to iropt.c
move Cmp(x,0) rules from opt_confirms.c to iropt.c, slightly improve it
only build shift instructions for two complement modes
do not change initial mem anchor when storing registerargs to stack