opt_aref_with is an optimized instruction for accessing a Hash using a
non-frozen string key (ie. from a file without frozen_string_literal).
It attempts to avoid allocating the string, and instead silently using a
frozen string (hash string keys are always fstrings).

Because this is just an optimization, it should be invisible to the
user. However, previously this optimization was could be seen via hashes
with default procs.

For example, previously:

    h = Hash.new { |h, k| k.frozen? }
    str = "foo"
    h[str]   # false
    h["foo"] # true when optimizations enabled

This commit checks that the Hash doesn't have a default proc when using
opt_aref_with.