[InstCombine] Remove bitcast handling from SimplifyDemandedBits

The complex set of type checks in this code reduces down to
"always return nullptr". Drop the code to use the default
implementation instead, which will just compute the KnownBits
for the bitcast.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -437,29 +437,6 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
     assert(!Known.hasConflict() && "Bits known to be one AND zero?");
     break;
   }
-  case Instruction::BitCast:
-    if (!I->getOperand(0)->getType()->isIntOrIntVectorTy())
-      return nullptr;  // vector->int or fp->int?
-
-    if (auto *DstVTy = dyn_cast<VectorType>(VTy)) {
-      if (auto *SrcVTy = dyn_cast<VectorType>(I->getOperand(0)->getType())) {
-        if (isa<ScalableVectorType>(DstVTy) ||
-            isa<ScalableVectorType>(SrcVTy) ||
-            cast<FixedVectorType>(DstVTy)->getNumElements() !=
-            cast<FixedVectorType>(SrcVTy)->getNumElements())
-          // Don't touch a bitcast between vectors of different element counts.
-          return nullptr;
-      } else
-        // Don't touch a scalar-to-vector bitcast.
-        return nullptr;
-    } else if (I->getOperand(0)->getType()->isVectorTy())
-      // Don't touch a vector-to-scalar bitcast.
-      return nullptr;
-
-    if (SimplifyDemandedBits(I, 0, DemandedMask, Known, Depth + 1))
-      return I;
-    assert(!Known.hasConflict() && "Bits known to be one AND zero?");
-    break;
   case Instruction::SExt: {
     // Compute the bits in the result that are not present in the input.
     unsigned SrcBitWidth = I->getOperand(0)->getType()->getScalarSizeInBits();

llvm/test/Transforms/InstCombine/icmp-vec.ll

@@ -649,9 +649,8 @@ define i1 @eq_cast_zext(<5 x i3> %b) {
 
 define i1 @sgt_cast_zext(<5 x i3> %b) {
 ; CHECK-LABEL: @sgt_cast_zext(
-; CHECK-NEXT:    [[E:%.*]] = zext <5 x i3> [[B:%.*]] to <5 x i7>
-; CHECK-NEXT:    [[BC:%.*]] = bitcast <5 x i7> [[E]] to i35
-; CHECK-NEXT:    [[R:%.*]] = icmp sgt i35 [[BC]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast <5 x i3> [[B:%.*]] to i15
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i15 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %e = zext <5 x i3> %b to <5 x i7>