aco: skip continue_or_break LCSSA phis when not needed
Fixes:
//exec is empty here
loop {
%1:s[16-17] = ...
if () {
break
}
%2:s[16-17] = ...
continue_or_break
}
%3 = phi %1, undef
//because of the undef, %2 can use s[16-17] and overwrite the address
load(%3:s[16-17])