stdlib: revert workaround for Span bounds checking

Author: eeckstein

stdlib/public/core/Span/MutableSpan.swift

@@ -290,18 +290,12 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   public subscript(_ position: Index) -> Element {
     unsafeAddress {
-      _precondition(
-        UInt(bitPattern: position) < UInt(bitPattern: _count),
-        "Index out of bounds"
-      )
+      _precondition(indices.contains(position), "index out of bounds")
       return unsafe UnsafePointer(_unsafeAddressOfElement(unchecked: position))
     }
     @lifetime(self: copy self)
     unsafeMutableAddress {
-      _precondition(
-        UInt(bitPattern: position) < UInt(bitPattern: _count),
-        "Index out of bounds"
-      )
+      _precondition(indices.contains(position), "index out of bounds")
        return unsafe _unsafeAddressOfElement(unchecked: position)
     }
   }

stdlib/public/core/Span/Span.swift

@@ -419,10 +419,7 @@ extension Span where Element: ~Copyable {
   @inline(__always)
   @_alwaysEmitIntoClient
   internal func _checkIndex(_ position: Index) {
-    _precondition(
-      UInt(bitPattern: position) < UInt(bitPattern: _count),
-      "Index out of bounds"
-    )
+    _precondition(indices.contains(position), "Index out of bounds")
   }
 
   /// Accesses the element at the specified position in the `Span`.