use correct svar

[ko1]

Without this patch, svar location is used "nearest Ruby frame". It is almost correct but it doesn't correct when the each method is written in Ruby.

class C
  include Enumerable
  def each
    %w(bar baz).each{|e| yield e}
  end
end

C.new.grep(/(b.)/){|e| p [$1, e]}

This patch fix this issue by traversing ifunc's cfp.

Note that if cfp doesn't specify this Thread's cfp stack, reserved svar location (ec->root_svar) is used.

[k0kubun]

requested a review from https://github.com/orgs/ruby/teams/yjit

Wrong ping. We did #7004 to skip this kind of PR, but it's disabled for now 74e52c2 because it had a bug.

---

The patch makes sense to me, thanks! This doesn't have a significant impact on performance either:

before: ruby 3.3.0dev (2023-02-01T21:05:22Z master 2675f2c864) [x86_64-linux]
last_commit=Remove whitespace
after: ruby 3.3.0dev (2023-02-01T23:59:25Z use_correct_ivar 7a81a4d59a) [x86_64-linux]

----------  -----------  ----------  ----------  ----------  ------------  -------------
bench       before (ms)  stddev (%)  after (ms)  stddev (%)  before/after  after 1st itr
railsbench  1551.1       0.6         1555.0      0.5         1.00          1.00
----------  -----------  ----------  ----------  ----------  ------------  -------------

I'll need this for #6687 and it doesn't hurt to correct this implementation, so let me merge this.