[SelectionDAG] Don't subject ISD:Constant to the depth limit in TargetLowering::SimplifyDemandedBits.

Summary:
We shouldn't recurse any further but it doesn't mean we shouldn't be able to give the known bits for a constant. The caller would probably like that we always return the right answer for a constant RHS. This matches what InstCombine does in this case.

I don't have a test case because this showed up while trying to revive D31724.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: arsenm, llvm-commits

Differential Revision: https://reviews.llvm.org/D38967

llvm-svn: 316255

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -516,6 +516,13 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
   // Don't know anything.
   Known = KnownBits(BitWidth);
 
+  if (Op.getOpcode() == ISD::Constant) {
+    // We know all of the bits for a constant!
+    Known.One = cast<ConstantSDNode>(Op)->getAPIntValue();
+    Known.Zero = ~Known.One;
+    return false;
+  }
+
   // Other users may use these bits.
   if (!Op.getNode()->hasOneUse() && !AssumeSingleUse) {
     if (Depth != 0) {
@@ -538,11 +545,6 @@ bool TargetLowering::SimplifyDemandedBits(SDValue Op,
 
   KnownBits Known2, KnownOut;
   switch (Op.getOpcode()) {
-  case ISD::Constant:
-    // We know all of the bits for a constant!
-    Known.One = cast<ConstantSDNode>(Op)->getAPIntValue();
-    Known.Zero = ~Known.One;
-    return false;   // Don't fall through, will infinitely loop.
   case ISD::BUILD_VECTOR:
     // Collect the known bits that are shared by every constant vector element.
     Known.Zero.setAllBits(); Known.One.setAllBits();