[llvm] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Simon Pilgrim via llvm-commits]

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@@ -213,6 +213,37 @@ TEST(KnownBitsTest, AddSubExhaustive) {
   TestAddSubExhaustive(false);
 }
 
+TEST(KnownBitsTest, SubBorrowExhaustive) {
+  unsigned Bits = 4;
+  ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
+    ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
+      ForeachKnownBits(1, [&](const KnownBits &KnownBorrow) {
+        // Explicitly compute known bits of the addition by trying all
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RKSimon wrote:

addition -> subtraction

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