[InstCombine] In SimplifyDemandedUseBits, use computeKnownBits directly to handle Constants

[craig.topper]

Summary

Currently we don't explicitly process ConstantDataSequential, ConstantAggregateZero, or ConstantVector, or Undef before applying the Depth limit. Instead they occur after the depth check in the non-instruction path.

For the constant types that we do handle, the code is replicated from computeKnownBits.

This patch fixes the missing constant handling and the reduces the amount of code by just using computeKnownBits directly for any type of Constant.

[spatel]

LGTM.

llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
   uint32_t BitWidth = DemandedMask.getBitWidth();
   Type *VTy = V->getType();
   assert(
       (!VTy->isIntOrIntVectorTy() || VTy->getScalarSizeInBits() == BitWidth) &&
       KnownZero.getBitWidth() == BitWidth &&
       KnownOne.getBitWidth() == BitWidth &&
       "Value *V, DemandedMask, KnownZero and KnownOne "
       "must have same BitWidth");
-  const APInt *C;
-  if (match(V, m_APInt(C))) {
-    // We know all of the bits for a scalar constant or a splat vector constant!
-    KnownOne = *C;
-    KnownZero = ~KnownOne;
-    return nullptr;
-  }
-  if (isa<ConstantPointerNull>(V)) {
-    // We know all of the bits for a constant!
-    KnownOne.clearAllBits();
-    KnownZero.setAllBits();
+
+  if (isa<Constant>(V)) {
+    computeKnownBits(V, KnownZero, KnownOne, Depth, CxtI);
     return nullptr;
   }
 
   KnownZero.clearAllBits();
   KnownOne.clearAllBits();
-  if (DemandedMask == 0) {   // Not demanding any bits from V.
-    if (isa<UndefValue>(V))
-      return nullptr;
+  if (DemandedMask == 0)     // Not demanding any bits from V.
     return UndefValue::get(VTy);
-  }
 
   if (Depth == 6)        // Limit search depth.
     return nullptr;
 
   Instruction *I = dyn_cast<Instruction>(V);
   if (!I) {
     computeKnownBits(V, KnownZero, KnownOne, Depth, CxtI);
     return nullptr;        // Only analyze instructions.