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.

[eregon]

We should remove the @ReportPolymorphism on SprintfNode above, it has no effect since there is a single specialization instance possible (I filed GR-53454 for Truffle to warn this).
Alternatively we could mark the method as Split.ALWAYS, I think that's a little better because then it just splits eagerly vs only when creating a second specialization instance of formatCached (and force splitting cannot be put on the wrong node like before this PR, unlike @ReportPolymorphism).

[eregon]

@andrykonchin Could you integrate this and add an extra commit removing the two @ReportPolymorphism and adding always split on the @CoreMethod?

It seems good to also check other cases of pack/unpack/format/sprintf/etc to make sure they are correct too.
One easy way to find them seems callers of global search RootCallTarget compile(.

[eregon]

On second thought, It seems good to keep the @ReportPolymorphism added in this PR, because then if someone does:

def myformat(format, *args)
  sprintf format, *args
end
myformat "%d", 1
myformat "%f", 3.14

it would correctly split, and wouldn't with only the always_split.

But let's always split SprintfNode since we know we'll want to split that one anyway, and the earlier the better for startup/warmup/footprint.