[NFC] Fix unary minus operator on unsigned type warning (#153887)

Fixes: `warning C4146: unary minus operator applied to unsigned type,
result still unsigned`

Author: Nadharm

llvm/include/llvm/IR/BasicBlock.h

@@ -68,7 +68,7 @@ class BasicBlock final : public Value, // Basic blocks are data objects also
   // Allow Function to renumber blocks.
   friend class Function;
   /// Per-function unique number.
-  unsigned Number = -1u;
+  unsigned Number = ~0u;
 
   friend class BlockAddress;
   friend class SymbolTableListTraits<BasicBlock>;