Remove usage of EqualityComparer from ValueTuple's GetHashCode

[jamesqo]

EqualityComparer<T>.Default.GetHashCode(item) has exactly the same behavior as item?.GetHashCode ?? 0. The only difference is that the latter version is faster because it is not an extra call.

Previously, the only compelling reason to use the former was that if item was an enum, then calling GetHashCode directly would box while EqualityComparer wouldn't (see here).

However, this change in coreclr and this one in corert eliminate boxing for enums, so there is no longer a reason to use EqualityComparer in ValueTuple. This PR replaces all usage of it with item?.GetHashCode() ?? 0. (I used a regex find-and-replace.)

cc @VSadov @jcouv @stephentoub @omariom

Fixes https://github.com/dotnet/corefx/issues/13774

[jkotas]

The corefx version of ValueTuple is really only going be just for existing runtimes (.NET Framework, .NET Core 1.0). The existing runtimes do not have the JIT optimization that this depends on.

@jcouv has a workitem to add the ValueTuple to CoreLib for CoreCLR. This change should wait until then, and it should be done only on the CoreLib copy.

[stephentoub]

Closing per @jkotas' comments. Thanks.

[jcouv]

FYI Adding ValueTuple to CoreLib for CoreCLR is tracked by umbrella issue dotnet/roslyn#13177

[jamesqo]

@jcouv Ok, thanks for telling me. I have posted a comment in that thread so everything is in one place.