[DAGCombiner] Remove (sra (shl X, C), C) if X has more than C sign bits.

If sext_inreg is supported, we will turn this into sext_inreg. That
will then remove it if there are enough sign bits. But if sext_inreg
isn't supported, we can still remove the shift pair based on sign
bits.

Split from D95890.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8368,6 +8368,10 @@ SDValue DAGCombiner::visitSRA(SDNode *N) {
         TargetLowering::Legal)
       return DAG.getNode(ISD::SIGN_EXTEND_INREG, SDLoc(N), VT,
                          N0.getOperand(0), DAG.getValueType(ExtVT));
+    // Even if we can't convert to sext_inreg, we might be able to remove
+    // this shift pair if the input is already sign extended.
+    if (DAG.ComputeNumSignBits(N0.getOperand(0)) > N1C->getZExtValue())
+      return N0.getOperand(0);
   }
 
   // fold (sra (sra x, c1), c2) -> (sra x, (add c1, c2))