YJIT: expandarray for non-arrays

[ywenc]

This PR updates the expandarray instruction for YJIT.

expandarray is an instruction that appears for array decomposition or multiple assignment.

Most commonly, expandarray will be encountered when assigning multiple variables at once from array,
example -

a, b, c = [1, 2, 3]

But it can often appear with non-arrays, when assigning multiple variables to a single value. It is common enough to see expandarray on a single nil that gen_expandarray previously had a special case if the operand was nil, but this is potentially incorrect because to_ary can be redefined on NilClass.

This PR updates the non-array case by bailing if to_ary is defined on a non-array operand. Otherwise, we push nils to fill the unset values, then push the original operand, which also helps get rid of side exits from a pattern like:

def some_method
   if ok
      true
   else 
      [false, err]
   end
end

value, err = some_method

This pattern was one of the most commonly seen side-exits for GitHub benchmarks.

Quick benchmark:

% time ./miniruby --disable-gems --yjit -e '10_000_000.times { a,b = 1 }'
./miniruby --disable-gems --yjit -e '10_000_000.times { a,b = 1 }'  0.35s user 0.02s system 61% cpu 0.594 total

% time ruby --disable-gems --yjit -e '10_000_000.times { a,b = 1 }'      
ruby --disable-gems --yjit -e '10_000_000.times { a,b = 1 }'  1.01s user 0.03s system 95% cpu 1.098 total