[Reassociate][NFC] Update the header comment of Reassociate Pass

[XChy]

Summary

The doc for reassociate has been outdated and inconsistent with its mechanism.
Its description about rank is wrong, which I update here.

[nikic]

llvm/lib/Transforms/Scalar/Reassociate.cpp

21

I think this comment shouldn't mention exact rank numbers. The important thing to convey at this level is that the rank ordering is constants, followed by arguments, followed by RPO order of instructions.

[XChy]

Emphasize ordering.

llvm/lib/Transforms/Scalar/Reassociate.cpp

 //===- Reassociate.cpp - Reassociate binary expressions -------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass reassociates commutative expressions in an order that is designed
 // to promote better constant propagation, GCSE, LICM, PRE, etc.
 //
 // For example: 4 + (x + 5) -> x + (4 + 5)
 //
-// In the implementation of this algorithm, constants are assigned rank = 0,
-// function arguments are rank = 1, and other values are assigned ranks
-// corresponding to the reverse post order traversal of current function
-// (starting at 2), which effectively gives values in deep loops higher rank
+// In the implementation of this algorithm, constants are assigned rank 0
+// firstly, the function arguments rank following constants, and at last, other
+// values are assigned ranks corresponding to the reverse post order traversal
+// of current function, which effectively gives values in deep loops higher rank
 // than values not in loops.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Scalar/Reassociate.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"