improve closure generation code

[felix winkelmann]

Large letrec forms in one example (genturfahi) generates code like this:

if(!C_stack_probe(&a)){
C_save_and_reclaim((void*)tr2,(void*)f_37755,2,t0,t1);}
t2=C_mutate(((C_word *)((C_word*)t0)[749])+1,t1);
t3=(*a=C_CLOSURE_TYPE|748,a[1]=(C_word)f_37759,a[2]=((C_word*)t0)[2],a[3]=((C_word*)t0)[3],a[4]=((C_word*)t0)[4],a[5]=((C_word*)t0)[5],a[6]=((C_word*)t0)[6],a[7]=((C_word*)t0)[7],a[8]=((C_word*)t0)[8],a[9]=((C_word*)t0)[9],a[10]=((C_word*)t0)[10],a[11]=((C_word*)t0)[11],a[12]=((C_word*)t0)[12],a[13]=((C_word*)t0)[13],a[14]=((C_word*)t0)[14],a[15]=((C_

...

[747],a[748]=((C_word*)t0)[748],tmp=(C_word)a,a+=749,tmp);
t4=(*a=C_CLOSURE_TYPE|2,a[1]=(C_word)f_82668,a[2]=t3,tmp=(C_word)a,a+=3,tmp);
t5=(*a=C_CLOSURE_TYPE|3,a[1]=(C_word)f_82670,a[2]=((C_word*)t0)[747],a[3]=((C_word)li5036),tmp=(C_word)a,a+=4,tmp);
t6=(*a=C_CLOSURE_TYPE|12,a[1]=(C_word)f_82678,a[2]=((C_word*)t0)[660],a[3]=((C_word*)t0)[659],a[4]=((C_word*)t0)[477],a[5]=((C_word*)t0)[635],a[6]=((C_word*)t0)[386],a[7]=t5,a[8]=t4,a[9]=((C_word*)t0)[697],a[10]=((C_word*)t0)[693],a[11]=((C_word*)t0)[712],a[12]=((C_word*)t0)[689],tmp=(C_word)a,a+=13,tmp);

Bad.