[clang-tools-extra] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Simon Pilgrim via cfe-commits]

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@@ -332,6 +332,10 @@ struct KnownBits {
   static KnownBits computeForAddSub(bool Add, bool NSW, const KnownBits &LHS,
                                     KnownBits RHS);
 
+  /// Compute known bits results from subtracting RHS from LHS.
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RKSimon wrote:

Add to comment "and a 1-bit borrow"

https://github.com/llvm/llvm-project/pull/67788