[llvm] [mlir] [ADT] Add implementations for mulhs and mulhu to APInt (PR #84609)

[Jakub Kuderski via llvm-commits]

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@@ -2193,6 +2193,12 @@ inline const APInt absdiff(const APInt &A, const APInt &B) {
   return A.uge(B) ? (A - B) : (B - A);
 }
 
+/// Compute the higher order bits of unsigned multiplication of two APInts
+APInt mulhu(const APInt &C1, const APInt &C2);
+
+/// Compute the higher order bits of signed multiplication of two APInts
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kuhar wrote:

Could you also add a formula similar to the one above? The main thing is to explain what happens with the sign bit IMO.

https://github.com/llvm/llvm-project/pull/84609