[llvm] [GlobalISel] Add G_ROTL/G_ROTR to computeNumSignBits (PR #213364)

[Simon Pilgrim via llvm-commits]

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@@ -485,6 +485,12 @@ struct KnownBits {
   /// Compute known bits for pdep(Val, Mask).
   LLVM_ABI static KnownBits pdep(const KnownBits &Val, const KnownBits &Mask);
 
+  /// Compute the number of sign bits after rotating a value.
+  LLVM_ABI static unsigned rotateNumSignBits(unsigned SrcSignBits,
+                                             unsigned BitWidth,
+                                             std::optional<uint64_t> RotAmt,
+                                             bool IsRotateRight);
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RKSimon wrote:

Do we have enough likely helpers (sharable SignBits code from ValueTracking/SelectionDAG/GISel) for it to be worth creating a llvm/Support/SignBits.h? I'm not convinced we need a SignBits struct, but a llvm::SignBitsOps namespace (similar to llvm::APIntOps) might be worth it if we have enough cases which aren't just the minimum signbits of the operands.....

@arsenm @nikic wdyt?

https://github.com/llvm/llvm-project/pull/213364