[llvm] [DAGCombiner] Attempt to fold 'add' nodes to funnel-shift or rotate (PR #125612)

[via llvm-commits]

Bhramar-vatsa wrote:

Thanks @AlexMaclean. We had a situation downstream which needed this transformation at SelectionDAG level too. I just wanted to mention that I tried following, similar to your change at IR level in [59ced72bc211f](https://github.com/llvm/llvm-project/commit/59ced72bc211f) (using the pattern matching framework at SD level).
```
static bool haveNoCommonBitsSetCommutative(SDValue A, SDValue B) {
  ...
+  SDValue V;
+  APInt R;
+    if (((sd_match(B, m_Shl(m_Value(), m_Sub(m_ConstInt(R), m_Value(V)))) &&
+          sd_match(A, m_Srl(m_Value(), m_Specific(V)))) ||
+         (sd_match(B, m_Srl(m_Value(), m_Sub(m_ConstInt(R), m_Value(V)))) &&
+          sd_match(A, m_Shl(m_Value(), m_Specific(V))))) &&
+        R.uge(A.getValueType().getScalarSizeInBits()))
+      return true;
      
  return false;
}
```
This then triggers the 'add' to 'or' transformation in [DAGCombiner](https://github.com/llvm/llvm-project/blob/dcb7a695004c49aaef02c3171343864870009961/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L2999), which in turn helps in the selection of the rotate instruction.

https://github.com/llvm/llvm-project/pull/125612