[DAGCombiner] Minor compile time improvement to (sext_in_reg (sign_extend_vector_inreg x)) optimization.

Don't bother calling ComputeNumSignBits if N00Bits < ExtVTBits. No
matter what answer we get back this will be true:
(N00Bits - DAG.ComputeNumSignBits(N00, DemandedSrcElts)) < ExtVTBits)

So we might as well save the computation. This makes the code more
consistent with the similar (sext_in_reg (sext x)) handling above.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11799,8 +11799,9 @@ SDValue DAGCombiner::visitSIGN_EXTEND_INREG(SDNode *N) {
     bool IsZext = N0.getOpcode() == ISD::ZERO_EXTEND_VECTOR_INREG;
     APInt DemandedSrcElts = APInt::getLowBitsSet(SrcElts, DstElts);
     if ((N00Bits == ExtVTBits ||
-         (!IsZext && (N00Bits - DAG.ComputeNumSignBits(N00, DemandedSrcElts)) <
-                         ExtVTBits)) &&
+         (!IsZext && (N00Bits < ExtVTBits ||
+                      (N00Bits - DAG.ComputeNumSignBits(N00, DemandedSrcElts)) <
+                          ExtVTBits))) &&
         (!LegalOperations ||
          TLI.isOperationLegal(ISD::SIGN_EXTEND_VECTOR_INREG, VT)))
       return DAG.getNode(ISD::SIGN_EXTEND_VECTOR_INREG, SDLoc(N), VT, N00);