This reverts commit a0980f2446c0db735b8ffeb37e241370c458a626.

Retry for macOS Mojave.

This reverts commit 9faef3113fb4331524b81ba73005ba13fa0ef6c6.

It seemed to cause a failure on macOS Mojave, though I'm unsure how.
https://rubyci.org/logs/rubyci.s3.amazonaws.com/osx1014/ruby-master/log/20190802T034503Z.fail.html.gz

This tentative revert is to check if the issue is actually caused by the
change or not.

This commit adds a specialized instruction for called to `.nil?`.  It is
about 27% faster than master in the case where the object is nil or not
nil.  In the case where an object implements `nil?`, I think it may be
slightly slower.  Here is a benchmark:

```ruby
require "benchmark/ips"

class Niller
  def nil?; true; end
end

not_nil = Object.new
xnil = nil
niller = Niller.new

Benchmark.ips do |x|
  x.report("nil?")    { xnil.nil? }
  x.report("not nil") { not_nil.nil? }
  x.report("niller")   { niller.nil? }
end
```

On Ruby master:

```
[aaron@TC ~/g/ruby (master)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   429.195k i/100ms
             not nil   437.889k i/100ms
              niller   437.935k i/100ms
Calculating -------------------------------------
                nil?     20.166M (± 8.1%) i/s -    100.002M in   5.002794s
             not nil     20.046M (± 7.6%) i/s -     99.839M in   5.020086s
              niller     22.467M (± 6.1%) i/s -    112.111M in   5.013817s
[aaron@TC ~/g/ruby (master)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   449.660k i/100ms
             not nil   433.836k i/100ms
              niller   443.073k i/100ms
Calculating -------------------------------------
                nil?     19.997M (± 8.8%) i/s -     99.375M in   5.020458s
             not nil     20.529M (± 7.0%) i/s -    102.385M in   5.020689s
              niller     21.796M (± 8.0%) i/s -    108.110M in   5.002300s
[aaron@TC ~/g/ruby (master)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   402.119k i/100ms
             not nil   438.968k i/100ms
              niller   398.226k i/100ms
Calculating -------------------------------------
                nil?     20.050M (±12.2%) i/s -     98.519M in   5.008817s
             not nil     20.614M (± 8.0%) i/s -    102.280M in   5.004531s
              niller     22.223M (± 8.8%) i/s -    110.309M in   5.013106s

```

On this branch:

```
[aaron@TC ~/g/ruby (specialized-nilp)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   468.371k i/100ms
             not nil   456.517k i/100ms
              niller   454.981k i/100ms
Calculating -------------------------------------
                nil?     27.849M (± 7.8%) i/s -    138.169M in   5.001730s
             not nil     26.417M (± 8.7%) i/s -    131.020M in   5.011674s
              niller     21.561M (± 7.5%) i/s -    107.376M in   5.018113s
[aaron@TC ~/g/ruby (specialized-nilp)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   477.259k i/100ms
             not nil   428.712k i/100ms
              niller   446.109k i/100ms
Calculating -------------------------------------
                nil?     28.071M (± 7.3%) i/s -    139.837M in   5.016590s
             not nil     25.789M (±12.9%) i/s -    126.470M in   5.011144s
              niller     20.002M (±12.2%) i/s -     98.144M in   5.001737s
[aaron@TC ~/g/ruby (specialized-nilp)]$ ./ruby compil.rb
Warming up --------------------------------------
                nil?   467.676k i/100ms
             not nil   445.791k i/100ms
              niller   415.024k i/100ms
Calculating -------------------------------------
                nil?     26.907M (± 8.0%) i/s -    133.755M in   5.013915s
             not nil     25.319M (± 7.9%) i/s -    125.713M in   5.007758s
              niller     19.569M (±11.8%) i/s -     96.286M in   5.008533s
```

Co-Authored-By: Ashe Connor <kivikakk@github.com>