Revert "Use Unsafe.BitCast for Int128 ↔UInt128 operators (#104506)" (#106430)

This reverts commit 01dbf51.

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/Int128.cs

@@ -425,7 +425,7 @@ public static explicit operator checked ulong(Int128 value)
         /// <param name="value">The value to convert.</param>
         /// <returns><paramref name="value" /> converted to a <see cref="UInt128" />.</returns>
         [CLSCompliant(false)]
-        public static explicit operator UInt128(Int128 value) => Unsafe.BitCast<Int128, UInt128>(value);
+        public static explicit operator UInt128(Int128 value) => new UInt128(value._upper, value._lower);
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="UInt128" /> value, throwing an overflow exception for any values that fall outside the representable range.</summary>
         /// <param name="value">The value to convert.</param>
@@ -438,7 +438,7 @@ public static explicit operator checked UInt128(Int128 value)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return Unsafe.BitCast<Int128, UInt128>(value);
+            return new UInt128(value._upper, value._lower);
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="UIntPtr" /> value.</summary>

src/libraries/System.Private.CoreLib/src/System/UInt128.cs

@@ -344,7 +344,7 @@ public static explicit operator checked long(UInt128 value)
         /// <param name="value">The value to convert.</param>
         /// <returns><paramref name="value" /> converted to a <see cref="Int128" />.</returns>
         [CLSCompliant(false)]
-        public static explicit operator Int128(UInt128 value) => Unsafe.BitCast<UInt128, Int128>(value);
+        public static explicit operator Int128(UInt128 value) => new Int128(value._upper, value._lower);
 
         /// <summary>Explicitly converts a 128-bit unsigned integer to a <see cref="Int128" /> value, throwing an overflow exception for any values that fall outside the representable range.</summary>
         /// <param name="value">The value to convert.</param>
@@ -357,7 +357,7 @@ public static explicit operator checked Int128(UInt128 value)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return Unsafe.BitCast<UInt128, Int128>(value);
+            return new Int128(value._upper, value._lower);
         }
 
         /// <summary>Explicitly converts a 128-bit unsigned integer to a <see cref="IntPtr" /> value.</summary>