iterator: improve type inference for Filter

This addresses type inference issues with filtered list comprehensions
reported in #59111. The root cause is the compiler's
inability to perform type refinement in cases like:

```julia let t::Tuple{Any,Any} x, y = t if x isa Int return t # still
`::Tuple{Any,Any}` end nothing end ```

Fixing this fundamentally would require extensive improvements to
abstract interpretation. As a workaround, I improved the iterators.jl
implementation to avoid this pattern.

One implementation consideration: if `iterate(f.itr, state...)` always
returns `Union{Nothing,Tuple{Any,Any}}`, the `if y isa Tuple{Any,Any}`
branch should be unnecessary. However, it's unclear whether we can
safely assume this for the iterate interface, so I kept the branch for
now.

Author: aviatesk

base/iterators.jl

@@ -534,10 +534,15 @@ filter(flt, itr) = Filter(flt, itr)
 function iterate(f::Filter, state...)
     y = iterate(f.itr, state...)
     while y !== nothing
-        if f.flt(y[1])
-            return y
+        v, s = y
+        if f.flt(v)
+            if y isa Tuple{Any,Any}
+                return (v, s) # incorporate type information that may be improved by user-provided `f.flt`
+            else
+                return y
+            end
         end
-        y = iterate(f.itr, y[2])
+        y = iterate(f.itr, s)
     end
     nothing
 end

test/iterators.jl

@@ -1201,3 +1201,8 @@ end
 @testset "Iterators docstrings" begin
     @test isempty(Docs.undocumented_names(Iterators))
 end
+
+# Filtered list comprehension (`Filter` construct) type inference
+@test Base.infer_return_type((Vector{Any},)) do xs
+    [x for x in xs if x isa Int]
+end == Vector{Int}