Use ComputeNumSignBits to fold (sext_inreg (trunc x)) -> (trunc x)

[vkalintiris]

Summary

If x's number of sign bits is greater than the number of sign bits
introduced by the sext_inreg, then we can skip the sign extension.

I can't think of any way to test this. I tried to using csmith without any
luck. This patch is intended for D16220.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
   // if x is small enough.
   if (N0.getOpcode() == ISD::SIGN_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND) {
     SDValue N00 = N0.getOperand(0);
     if (N00.getValueType().getScalarType().getSizeInBits() <= EVTBits &&
         (!LegalOperations || TLI.isOperationLegal(ISD::SIGN_EXTEND, VT)))
       return DAG.getNode(ISD::SIGN_EXTEND, SDLoc(N), VT, N00, N1);
   }
 
+  // fold (sext_in_reg (trunc x)) -> (trunc x) if x's sign bits start from
+  // EVTBits.
+  if (N0.getOpcode() == ISD::TRUNCATE) {
+    unsigned N00VTBits = N0.getOperand(0).getValueType().getSizeInBits();
+    unsigned N00NSB = DAG.ComputeNumSignBits(N0.getOperand(0));
+
+    if ((N00VTBits - N00NSB + 1) <= EVTBits)
+      return N0;
+  }
+
   // fold (sext_in_reg x) -> (zext_in_reg x) if the sign bit is known zero.
   if (DAG.MaskedValueIsZero(N0, APInt::getBitsSet(VTBits, EVTBits-1, EVTBits)))
     return DAG.getZeroExtendInReg(N0, SDLoc(N), EVT);
 
   // fold operands of sext_in_reg based on knowledge that the top bits are not
   // demanded.
   if (SimplifyDemandedBits(SDValue(N, 0)))
     return SDValue(N, 0);