scriptcomp: Fold equivalent instructions

[mtijanic]

Sample script

void main() {
    if (1) {
        string s = "AAA";
    }
}

gets compiled down to

     0  JSR, 8                                  
     6  RET                                     
     8  CONSTANT, TYPE_INTEGER, 1               
    14  JZ, 41                                  
    20  RUNSTACK_ADD, TYPE_STRING               
    22  CONSTANT, TYPE_STRING, "AAA"            
    29  ASSIGNMENT, TYPE_VOID, -8, 4            
    37  MODIFY_STACK_POINTER, -4                
    43  MODIFY_STACK_POINTER, -4                
    49  JMP, 6                                  
    55  RET         

the two MODIFY_STACK_POINTER instructions could be a single one. There are other such instances.

I think for this to work, we probably need another pass over the generated NCS before writing it out, and then fixing up the jumps.
Alternatively, we can just keep track of the last instruction, and if it is the same as current, we modify it instead.