[stdlib] relax stride check

- The stride check in `UnsafePointer.withMemoryRebound` makes less sense when rebinding memory for a single element.
- This skips the stride-matching portion of the `_debugPrecondition` in `withMemoryRebound` when `count == 1`.

Author: glessard

stdlib/public/core/UnsafePointer.swift

@@ -315,9 +315,10 @@ public struct UnsafePointer<Pointee>: _Pointer {
   ) rethrows -> Result {
     _debugPrecondition(
       Int(bitPattern: .init(_rawValue)) & (MemoryLayout<T>.alignment-1) == 0 &&
-      MemoryLayout<Pointee>.stride > MemoryLayout<T>.stride
-      ? MemoryLayout<Pointee>.stride % MemoryLayout<T>.stride == 0
-      : MemoryLayout<T>.stride % MemoryLayout<Pointee>.stride == 0
+      ( count == 1 ||
+        MemoryLayout<Pointee>.stride > MemoryLayout<T>.stride
+        ? MemoryLayout<Pointee>.stride % MemoryLayout<T>.stride == 0
+        : MemoryLayout<T>.stride % MemoryLayout<Pointee>.stride == 0 )
     )
     let binding = Builtin.bindMemory(_rawValue, count._builtinWordValue, T.self)
     defer { Builtin.rebindMemory(_rawValue, binding) }
@@ -1001,9 +1002,10 @@ public struct UnsafeMutablePointer<Pointee>: _Pointer {
   ) rethrows -> Result {
     _debugPrecondition(
       Int(bitPattern: .init(_rawValue)) & (MemoryLayout<T>.alignment-1) == 0 &&
-      MemoryLayout<Pointee>.stride > MemoryLayout<T>.stride
-      ? MemoryLayout<Pointee>.stride % MemoryLayout<T>.stride == 0
-      : MemoryLayout<T>.stride % MemoryLayout<Pointee>.stride == 0
+      ( count == 1 ||
+        MemoryLayout<Pointee>.stride > MemoryLayout<T>.stride
+        ? MemoryLayout<Pointee>.stride % MemoryLayout<T>.stride == 0
+        : MemoryLayout<T>.stride % MemoryLayout<Pointee>.stride == 0 )
     )
     let binding = Builtin.bindMemory(_rawValue, count._builtinWordValue, T.self)
     defer { Builtin.rebindMemory(_rawValue, binding) }