Reduce begin-less/end-less range allocation

```
$ cat test.yaml
prelude: |
    def endless
      1..
    end

    def beginless
      ..1
    end

    def endless_substr(str)
      str[1..]
    end

benchmark:
    endless: endless
    beginless: beginless
    endless_substr: "endless_substr('foo')"

$ RBENV_VERSION=trunk ruby -v
ruby 2.8.0dev (2020-02-15T12:52:03Z master 961630126b) [x86_64-linux]
$ RBENV_VERSION=patched ruby -v
ruby 2.8.0dev (2020-02-15T12:52:03Z origin/master 961630126b) [x86_64-linux]

$ benchmark-driver test.yaml --rbenv 'patched;trunk'
Warming up --------------------------------------
             endless    45.948M i/s -     46.076M times in 1.002782s (21.76ns/i, 26clocks/i)
           beginless    49.986M i/s -     50.237M times in 1.005037s (20.01ns/i, 24clocks/i)
      endless_substr     8.067M i/s -      8.187M times in 1.014936s (123.96ns/i, 148clocks/i)
Calculating -------------------------------------
                        patched       trunk
             endless   115.679M     21.500M i/s -    137.843M times in 1.191597s 6.411398s
           beginless   112.599M     22.060M i/s -    149.957M times in 1.331778s 6.797768s
      endless_substr     8.888M      6.760M i/s -     24.201M times in 2.722995s 3.580038s

Comparison:
                          endless
             patched: 115679391.9 i/s
               trunk:  21499711.2 i/s - 5.38x  slower

                        beginless
             patched: 112598731.5 i/s
               trunk:  22059673.0 i/s - 5.10x  slower

                   endless_substr
             patched:   8887513.1 i/s
               trunk:   6759886.2 i/s - 1.31x  slower
```

compile.c

@@ -5178,9 +5178,9 @@ compile_named_capture_assign(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE
 }
 
 static int
-number_literal_p(const NODE *n)
+optimizable_range_item_p(const NODE *n)
 {
-    return (n && nd_type(n) == NODE_LIT && RB_INTEGER_TYPE_P(n->nd_lit));
+    return (n && nd_type(n) == NODE_LIT && RB_INTEGER_TYPE_P(n->nd_lit)) || (n && nd_type(n) == NODE_NIL);
 }
 
 static int
@@ -8307,10 +8307,12 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 	VALUE flag = INT2FIX(excl);
 	const NODE *b = node->nd_beg;
 	const NODE *e = node->nd_end;
-	if (number_literal_p(b) && number_literal_p(e)) {
+	if (optimizable_range_item_p(b) && optimizable_range_item_p(e)) {
 	    if (!popped) {
-		VALUE val = rb_range_new(b->nd_lit, e->nd_lit, excl);
+                VALUE bv = nd_type(b) == NODE_LIT ? b->nd_lit : Qnil;
-		ADD_INSN1(ret, line, putobject, val);
+                VALUE ev = nd_type(e) == NODE_LIT ? e->nd_lit : Qnil;
+                VALUE val = rb_range_new(bv, ev, excl);
+                ADD_INSN1(ret, line, putobject, val);
                 RB_OBJ_WRITTEN(iseq, Qundef, val);
 	    }
 	}