fix is_signed to use proper Julia type hierarchy check

- T <: Integer && !(T <: Unsigned) correctly identifies:
  - Int32, Int64, etc. as signed (true)
  - UInt32, UInt64, etc. as unsigned (false)

Author: arhik

src/compiler/intrinsics/core.jl

@@ -671,7 +671,7 @@ end
 
 Return true if type T is signed, false for unsigned types.
 """
-is_signed(::Type{T}) where T <: Integer = T <: SignedInteger
+is_signed(::Type{T}) where T <: Integer = T <: Integer && !(T <: Unsigned)
 is_signed(::Type{T}) where T <: AbstractFloat = false
 
 """