IR: Add a shufflevector mask commutation helper function. NFC.

[zvi]

Summary

Following up on Sanjay's suggetion in D32955, move this functionality
into ShuffleVectornstruction.

[RKSimon]

LGTM

include/llvm/IR/Instructions.h

public:
   }
 
   SmallVector<int, 16> getShuffleMask() const {
     SmallVector<int, 16> Mask;
     getShuffleMask(Mask);
     return Mask;
   }
 
+  /// Change values in a shuffle permute mask assuming the two vector operands
+  /// of length InVecNumElts have swapped position.
+  static void commuteShuffleMask(MutableArrayRef<int> Mask,
+                                 unsigned InVecNumElts) {
+    for (int &Idx : Mask) {
+      if (Idx == -1)
+        continue;
+      Idx = Idx < (int)InVecNumElts ? Idx + InVecNumElts : Idx - InVecNumElts;
+      assert(Idx >= 0 && Idx < (int)InVecNumElts * 2 &&
+             "shufflevector mask index out of range");
+    }
+  }
+
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static inline bool classof(const Instruction *I) {
     return I->getOpcode() == Instruction::ShuffleVector;
   }
   static inline bool classof(const Value *V) {
     return isa<Instruction>(V) && classof(cast<Instruction>(V));
   }
 };

lib/Analysis/InstructionSimplify.cpp

static Value *SimplifyShuffleVectorInst(Value *Op0, Value *Op1, Constant *Mask,
   // If all operands are constant, constant fold the shuffle.
   if (Op0Const && Op1Const)
     return ConstantFoldShuffleVectorInstruction(Op0Const, Op1Const, Mask);
 
   // Canonicalization: if only one input vector is constant, it shall be the
   // second one.
   if (Op0Const && !Op1Const) {
     std::swap(Op0, Op1);
-    for (int &Idx : Indices) {
-      if (Idx == -1)
-        continue;
-      Idx = Idx < (int)InVecNumElts ? Idx + InVecNumElts : Idx - InVecNumElts;
-      assert(Idx >= 0 && Idx < (int)InVecNumElts * 2 &&
-             "shufflevector mask index out of range");
-    }
+    ShuffleVectorInst::commuteShuffleMask(Indices, InVecNumElts);
     Mask = ConstantDataVector::get(
         Mask->getContext(),
         makeArrayRef(reinterpret_cast<uint32_t *>(Indices.data()),
                      MaskNumElts));
   }
 
   // A shuffle of a splat is always the splat itself. Legal if the shuffle's
   // value type is same as the input vectors' type.

unittests/IR/InstructionsTest.cpp

 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/NoFolder.h"
 #include "llvm/IR/Operator.h"
+#include "gmock/gmock-matchers.h"
 #include "gtest/gtest.h"
 #include <memory>
 
 namespace llvm {
 namespace {
 
 TEST(InstructionsTest, ReturnInst) {
   LLVMContext C;
TEST(InstructionsTest, SwitchInst) {
   EXPECT_EQ(3, std::distance(CCI, CCE));
 
   // Make sure that the const iterator is compatible with a const auto ref.
   const auto &Handle = *CCI;
   EXPECT_EQ(1, Handle.getCaseValue()->getSExtValue());
   EXPECT_EQ(BB1.get(), Handle.getCaseSuccessor());
 }
 
+TEST(InstructionsTest, CommuteShuffleMask) {
+  SmallVector<int, 16> Indices({-1, 0, 7});
+  ShuffleVectorInst::commuteShuffleMask(Indices, 4);
+  EXPECT_THAT(Indices, testing::ContainerEq(ArrayRef<int>({-1, 4, 3})));
+}
+
 } // end anonymous namespace
 } // end namespace llvm