Report polymorphism for sprintf nodes.

[nirvdrum]

While looking at the performance of a large Rails application, I saw that we were creating new CallTarget on each call to String#%. The generic specialization compiles the sprintf expression from scratch on each call, resulting in the creation of a new call target.

The sprintf code can be invoked in a few different ways, but the one that stood out to me was String#%. Rails will create a request ID for each request to make tracking logs and such easier. The ActiveSupport code for creating the UUID uses a static format string, but the sprintf nodes are already megamorphic by the time this code is called. I saw it go megamorphic by loading the URI library.

I suspect format strings are mostly static and splitting would make most call sites monomorphic. This simple change demonstrably splits with the following example:

def foo(format)
 format % [123]
end

loop do
  foo "%#{rand(3)}d"
  foo "%s"
end

For call sites with > 3 format strings we could add a global cache, like we do for regular expressions. That could cut down on the creation of unique call targets, but that's out of scope for this PR and I don't have any evidence of it being a real world problem at the moment.

[nirvdrum]

I'm sorry. I noticed a typo in the branch name and used GitHub's UI for renaming the branch. I thought GitHub would treat that more intelligently than manually deleting the branch. But, it did not and when GitHub deleted the branch, it closed this PR. I've reopened as #3537.