Revert "Optimize BOX+UNBOX for "T? -> T" and "T -> T?" (#104931)"

[ericstj]

This reverts commit 9df4f7f. #104931

This change was causing failures in EF Core:
dotnet/efcore#34278 (comment)
https://dev.azure.com/dnceng-public/public/_build/results?buildId=754823&view=results

Microsoft.Data.Sqlite.SqliteException : SQLite Error 1: 'The JIT compiler encountered invalid IL code or an internal limitation.'.
Stack trace
   at Microsoft.Data.Sqlite.SqliteException.ThrowExceptionForRC(Int32 rc, sqlite3 db) in D:\a\_work\1\s\src\Microsoft.Data.Sqlite.Core\SqliteException.cs:line 84
   at Microsoft.Data.Sqlite.SqliteDataReader.NextResult() in D:\a\_work\1\s\src\Microsoft.Data.Sqlite.Core\SqliteDataReader.cs:line 175
   at Microsoft.Data.Sqlite.SqliteCommand.ExecuteReader(CommandBehavior behavior) in D:\a\_work\1\s\src\Microsoft.Data.Sqlite.Core\SqliteCommand.cs:line 312
   at Microsoft.Data.Sqlite.SqliteCommand.ExecuteDbDataReader(CommandBehavior behavior) in D:\a\_work\1\s\src\Microsoft.Data.Sqlite.Core\SqliteCommand.cs:line 355
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReader(RelationalCommandParameterObject parameterObject) in D:\a\_work\1\s\src\EFCore.Relational\Storage\RelationalCommand.cs:line 530
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.InitializeReader(Enumerator enumerator) in D:\a\_work\1\s\src\EFCore.Relational\Query\Internal\SingleQueryingEnumerable.cs:line 266
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.<>c.<MoveNext>b__21_0(DbContext _, Enumerator enumerator) in D:\a\_work\1\s\src\EFCore.Relational\Query\Internal\SingleQueryingEnumerable.cs:line 199
   at Microsoft.EntityFrameworkCore.Storage.NonRetryingExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded) in D:\a\_work\1\s\src\EFCore\Storage\NonRetryingExecutionStrategy.cs:line 75
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.MoveNext() in D:\a\_work\1\s\src\EFCore.Relational\Query\Internal\SingleQueryingEnumerable.cs:line 194
   at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Boolean& found)
   at lambda_method18058(Closure, QueryContext)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteCore[TResult](Expression query, Boolean async, CancellationToken cancellationToken) in D:\a\_work\1\s\src\EFCore\Query\Internal\QueryCompiler.cs:line 79
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query) in D:\a\_work\1\s\src\EFCore\Query\Internal\QueryCompiler.cs:line 60
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression) in D:\a\_work\1\s\src\EFCore\Query\Internal\EntityQueryProvider.cs:line 64
   at Microsoft.EntityFrameworkCore.BuiltInDataTypesSqliteTest.Can_query_less_than_of_converted_types() in D:\a\_work\1\s\test\EFCore.Sqlite.FunctionalTests\BuiltInDataTypesSqliteTest.cs:line 1125
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)

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[lewing]

fix for errors in dotnet/efcore#34278

[ericstj]

I needed to add back eeIsSharedInst as that was used by f8bcd05

[ericstj]

Treating as "tell-mode" to unblock preview7.

[carlossanlop]

If it's a direct revert, would you like me to fast-merge it?

Edit: Nevermind, I see the extra commit. But let me know anyway if you'd like it merged asap.

[EgorBo]

Fixed in Main via #105529