JIT: Normalizing 32 -> 64 bit zero extension representations

[jakobbotsch]

These zero extensions are marked with GTF_UNSIGNED, but we create them both with gtCastType = TYP_ULONG and gtCastType = TYP_LONG. We should settle on just one representation. See #122184 for more context.

cc @dotnet/jit-contrib

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.