Don't widen to Int128 in CastCache (#92984)

Leftover change from hackathon. Existing code is widening to Int128 and then counting zeros. It still worked because we only use it for bit shifting and bit shifts do wrap around if they're > 64. But this was bringing Int128 into `Runtime.Base` in my hackathon project.

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastCache.cs

@@ -250,7 +250,7 @@ internal static CastResult TryGet(int[] table, nuint source, nuint target)
  TableMask(ref tableData) = size - 1;
 
  // Fibonacci hash reduces the value into desired range by shifting right by the number of leading zeroes in 'size-1'
- byte shift = (byte)BitOperations.LeadingZeroCount(size - 1);
+ byte shift = (byte)BitOperations.LeadingZeroCount((nuint)(size - 1));
  HashShift(ref tableData) = shift;
 
  return table;

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/GenericCache.cs

@@ -230,7 +230,7 @@ internal bool TryGet(TKey key, out TValue? value)
  ref Entry tableData = ref TableData(table);
 
  // Fibonacci hash reduces the value into desired range by shifting right by the number of leading zeroes in 'size-1'
- byte shift = (byte)BitOperations.LeadingZeroCount(size - 1);
+ byte shift = (byte)BitOperations.LeadingZeroCount((nuint)(size - 1));
  HashShift(table) = shift;
 
  return table;