optimizations: improve Core._apply_iterate call conversion in #59548 (#59601)

Further improves the implementation from #59548.
Specifically, uses `widenconst` to enable conversion of `tuple` calls
that have become `PartialStruct`, and removes incorrect comments and
unused arguments.

Also adds some Julia-IR level tests.

(cherry picked from commit a4e02ca)

Author: aviatesk

Compiler/test/irpasses.jl

@@ -2059,3 +2059,17 @@ let src = code_typed1((Vector{Any},)) do xs
     end
     @test count(iscall((src, Core.svec)), src.code) == 1
 end
+
+# JuliaLang/julia #59548
+# Rewrite `Core._apply_iterate` to use `Core.svec` instead of `tuple` to better match
+# the codegen ABI
+let src = code_typed1((Vector{Any},)) do xs
+        println(stdout, xs...)
+    end
+    @test count(iscall((src, Core.svec)), src.code) == 1
+end
+let src = code_typed1((Vector{Any},)) do xs
+        println(stdout, 1, xs...) # convert tuples represented by `PartialStruct`
+    end
+    @test count(iscall((src, Core.svec)), src.code) == 1
+end