Optimization of REXML::Parent#index

[naitoh]

Benchmark

$ benchmark-driver benchmark/xpath.yaml
                                                                           before       after  before(YJIT)  after(YJIT)
                  REXML::XPath.match(REXML::Document.new(xml), 'a//a')     4.016k      4.274k        3.942k       4.106k i/s -     100.000 times in 0.024899s 0.023398s 0.025371s 0.024357s
                REXML::XPath.match(REXML::Document.new(xml), '//a//a')     1.327k      1.435k        1.659k       1.742k i/s -     100.000 times in 0.075383s 0.069665s 0.060272s 0.057402s
   REXML::Document.new(xml_wide).root.children.first.next_sibling_node     5.883M      7.144M      219.780k     279.330k i/s -     100.000 times in 0.000017s 0.000014s 0.000455s 0.000358s
REXML::Document.new(xml_wide).root.children.last.previous_sibling_node   109.051k    207.900k       97.466k     139.276k i/s -     100.000 times in 0.000917s 0.000481s 0.001026s 0.000718s

Comparison:
                               REXML::XPath.match(REXML::Document.new(xml), 'a//a')
                                                                 after:      4273.9 i/s
                                                           after(YJIT):      4105.6 i/s - 1.04x  slower
                                                                before:      4016.2 i/s - 1.06x  slower
                                                          before(YJIT):      3941.5 i/s - 1.08x  slower

                             REXML::XPath.match(REXML::Document.new(xml), '//a//a')
                                                           after(YJIT):      1742.1 i/s
                                                          before(YJIT):      1659.1 i/s - 1.05x  slower
                                                                 after:      1435.4 i/s - 1.21x  slower
                                                                before:      1326.6 i/s - 1.31x  slower

                REXML::Document.new(xml_wide).root.children.first.next_sibling_node
                                                                 after:   7143515.6 i/s
                                                                before:   5882549.8 i/s - 1.21x  slower
                                                           after(YJIT):    279330.0 i/s - 25.57x  slower
                                                          before(YJIT):    219780.0 i/s - 32.50x  slower

             REXML::Document.new(xml_wide).root.children.last.previous_sibling_node
                                                                 after:    207900.1 i/s
                                                           after(YJIT):    139275.9 i/s - 1.49x  slower
                                                                before:    109051.2 i/s - 1.91x  slower
                                                          before(YJIT):     97465.8 i/s - 2.13x  slower

• YJIT=ON : 1.04x - 1.42x faster
• YJIT=OFF : 1.06x - 1.91x faster