[ADT] Simplify PointerBitMask in PointerIntPair.h (NFC) (#158210)

A left shift of (uintptr_t)-1) is simpler.

Author: kazutakahirata

llvm/include/llvm/ADT/PointerIntPair.h

@@ -173,8 +173,7 @@ struct PointerIntPairInfo {
                 "PointerIntPair with integer size too large for pointer");
   enum MaskAndShiftConstants : uintptr_t {
     /// PointerBitMask - The bits that come from the pointer.
-    PointerBitMask =
-        ~(uintptr_t)(((intptr_t)1 << PtrTraits::NumLowBitsAvailable) - 1),
+    PointerBitMask = (~(uintptr_t)0) << PtrTraits::NumLowBitsAvailable,
 
     /// IntShift - The number of low bits that we reserve for other uses, and
     /// keep zero.