[InstCombine] [NFC] Use a pattern matcher for ExtractElementInst.

[RKSimon]

LGTM

[Chenbing.Zheng]

I think this is a nfc patch, but I find there are two failed tests AMDGPU::vector-alloca-bitcast.ll and AMDGPU::vector-alloca.ll. I do not have an AMDGPU machine， so I can not debug it.

[Chenbing.Zheng]

I got it, Index may be a variable，use m_value() instead of m_ConstantInt().

[rampitec]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

 }
 
 /// Canonicalize scalar bitcasts of extracted elements into a bitcast of the
 /// vector followed by extract element. The backend tends to handle bitcasts of
 /// vectors better than bitcasts of scalars because vector registers are
 /// usually not type-specific like scalar integer or scalar floating-point.
 static Instruction *canonicalizeBitCastExtElt(BitCastInst &BitCast,
                                               InstCombinerImpl &IC) {
-  // TODO: Create and use a pattern matcher for ExtractElementInst.
-  auto *ExtElt = dyn_cast<ExtractElementInst>(BitCast.getOperand(0));
-  if (!ExtElt || !ExtElt->hasOneUse())
+  Value *VecOp, *Index;
+  if (!match(BitCast.getOperand(0),
+             m_OneUse(m_ExtractElt(m_Value(VecOp), m_Value(Index)))))
     return nullptr;
 
   // The bitcast must be to a vectorizable type, otherwise we can't make a new
   // type to extract from.
   Type *DestType = BitCast.getType();
   if (!VectorType::isValidElementType(DestType))
     return nullptr;
 
-  auto *NewVecType = VectorType::get(DestType, ExtElt->getVectorOperandType());
-  auto *NewBC = IC.Builder.CreateBitCast(ExtElt->getVectorOperand(),
-                                         NewVecType, "bc");
-  return ExtractElementInst::Create(NewBC, ExtElt->getIndexOperand());
+  auto *NewVecType =
+      VectorType::get(DestType, cast<VectorType>(VecOp->getType()));
+  auto *NewBC = IC.Builder.CreateBitCast(VecOp, NewVecType, "bc");
+  return ExtractElementInst::Create(NewBC, Index);
 }
 
 /// Change the type of a bitwise logic operation if we can eliminate a bitcast.
 static Instruction *foldBitCastBitwiseLogic(BitCastInst &BitCast,
                                             InstCombiner::BuilderTy &Builder) {
   Type *DestTy = BitCast.getType();
   BinaryOperator *BO;
   if (!DestTy->isIntOrIntVectorTy() ||