Make opt_aref instruction support Integer#[]

only when its receiver and the argument are both Integers.

Since 6bedbf4625, Integer#[] has supported a range extraction.
This means that Integer#[] now accepts multiple arguments, which made
the method very slow unfortunately.

This change fixes the performance issue by adding a special handling for
its traditional use case: `num[idx]` where both `num` and `idx` are
Integers.

internal.h

@@ -1737,6 +1737,7 @@ VALUE rb_int_round(VALUE num, int ndigits, enum ruby_num_rounding_mode mode);
 VALUE rb_int2str(VALUE num, int base);
 VALUE rb_dbl_hash(double d);
 VALUE rb_fix_plus(VALUE x, VALUE y);
+VALUE rb_fix_aref(VALUE fix, VALUE idx);
 VALUE rb_int_gt(VALUE x, VALUE y);
 int rb_float_cmp(VALUE x, VALUE y);
 VALUE rb_float_gt(VALUE x, VALUE y);

numeric.c

@@ -4630,8 +4630,8 @@ rb_int_rshift(VALUE x, VALUE y)
     return Qnil;
 }
 
-static VALUE
-fix_aref(VALUE fix, VALUE idx)
+MJIT_FUNC_EXPORTED VALUE
+rb_fix_aref(VALUE fix, VALUE idx)
 {
     long val = FIX2LONG(fix);
     long i;
@@ -4722,7 +4722,7 @@ int_aref1(VALUE num, VALUE arg)
 
 one_bit:
     if (FIXNUM_P(num)) {
-        return fix_aref(num, arg);
+        return rb_fix_aref(num, arg);
     }
     else if (RB_TYPE_P(num, T_BIGNUM)) {
         return rb_big_aref(num, arg);

vm.c

@@ -1643,7 +1643,7 @@ vm_init_redefined_flag(void)
     OP(GT, GT), (C(Integer), C(Float));
     OP(GE, GE), (C(Integer), C(Float));
     OP(LTLT, LTLT), (C(String), C(Array));
-    OP(AREF, AREF), (C(Array), C(Hash));
+    OP(AREF, AREF), (C(Array), C(Hash), C(Integer));
     OP(ASET, ASET), (C(Array), C(Hash));
     OP(Length, LENGTH), (C(Array), C(String), C(Hash));
     OP(Size, SIZE), (C(Array), C(String), C(Hash));

vm_insnhelper.c

@@ -4129,6 +4129,10 @@ static VALUE
 vm_opt_aref(VALUE recv, VALUE obj)
 {
     if (SPECIAL_CONST_P(recv)) {
+	if (FIXNUM_P(recv) && FIXNUM_P(obj) &&
+		BASIC_OP_UNREDEFINED_P(BOP_AREF, INTEGER_REDEFINED_OP_FLAG)) {
+	    return rb_fix_aref(recv, obj);
+	}
 	return Qundef;
     }
     else if (RBASIC_CLASS(recv) == rb_cArray &&