[llvm] [KnownBits] Check that mul is optimal for low order bits (PR #113316)

[Ramkumar Ramachandra via llvm-commits]

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@@ -815,4 +815,38 @@ TEST(KnownBitsTest, ConcatBits) {
   }
 }
 
+TEST(KnownBitsTest, MulExhaustive) {
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artagnon wrote:

Shouldn't this be called `MulLowerBitsExhaustive` or something?

https://github.com/llvm/llvm-project/pull/113316