range.c: optimize rb_range_last for int renage

This change improves the performance of Range#last method for a range
of integer.  This optimization directly computes only required values
only in a range instead of converting all values in the range to an
array.  The optimized implementation is 129-16.7k times faster than
the previous one in the benchmark result given below.

=== Benchmark Result ===

```
$ make benchmark ITEM=range_last COMPARE_RUBY=/Users/mrkn/.rbenv/versions/2.6.0/bin/ruby
generating known_errors.inc
known_errors.inc unchanged
/Users/mrkn/src/github.com/ruby/ruby/revision.h unchanged
/Users/mrkn/.rbenv/shims/ruby --disable=gems -rrubygems -I/Users/mrkn/src/github.com/ruby/ruby/benchmark/lib /Users/mrkn/src/github.com/ruby/ruby/benchmark/benchmark-driver/exe/benchmark-driver \
                    --executables="compare-ruby::/Users/mrkn/.rbenv/versions/2.6.0/bin/ruby -I.ext/common --disable-gem" \
                    --executables="built-ruby::./miniruby -I/Users/mrkn/src/github.com/ruby/ruby/lib -I. -I.ext/common  -r/Users/mrkn/src/github.com/ruby/ruby/prelude --disable-gem" \
                    $(find /Users/mrkn/src/github.com/ruby/ruby/benchmark -maxdepth 1 -name '*range_last*.yml' -o -name '*range_last*.rb' | sort)
Warming up --------------------------------------
  (1..1_000_000).last(100)       35.600 i/s -      36.000 times in 1.011239s (28.09ms/i)
 (1..1_000_000).last(1000)       36.204 i/s -      39.000 times in 1.077240s (27.62ms/i)
(1..1_000_000).last(10000)       36.470 i/s -      39.000 times in 1.069386s (27.42ms/i)
Calculating -------------------------------------
                           compare-ruby  built-ruby
  (1..1_000_000).last(100)       36.477    609.196k i/s -     106.000 times in 2.905950s 0.000174s
 (1..1_000_000).last(1000)       35.936     50.350k i/s -     108.000 times in 3.005321s 0.002145s
(1..1_000_000).last(10000)       35.641      4.602k i/s -     109.000 times in 3.058233s 0.023685s

Comparison:
               (1..1_000_000).last(100)
                built-ruby:    609195.7 i/s
              compare-ruby:        36.5 i/s - 16700.87x  slower

              (1..1_000_000).last(1000)
                built-ruby:     50349.7 i/s
              compare-ruby:        35.9 i/s - 1401.08x  slower

             (1..1_000_000).last(10000)
                built-ruby:      4602.1 i/s
              compare-ruby:        35.6 i/s - 129.12x  slower
```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66734 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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