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

[Simon Pilgrim via llvm-commits]

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@@ -1446,3 +1446,26 @@ LLVM_DUMP_METHOD void KnownBits::dump() const {
   dbgs() << "\n";
 }
 #endif
+
+unsigned llvm::rotateNumSignBits(unsigned SrcSignBits, unsigned BitWidth,
+                                 std::optional<uint64_t> RotAmt,
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RKSimon wrote:

I'm tempted to suggest we use either `std::optional<APInt> RotAmt` or even `const KnownBits &RotAmt` arg here 

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