[LLVM-C] Turn a ShuffleVector Constant Into a Getter.

It is not a good idea to expose raw constants in the LLVM C API. Replace this with an explicit getter.

Differential Revision: https://reviews.llvm.org/D88367

(cherry picked from commit 55f7273)

llvm/include/llvm-c/Core.h

@@ -3942,13 +3942,20 @@ LLVMValueRef LLVMBuildAtomicCmpXchg(LLVMBuilderRef B, LLVMValueRef Ptr,
  */
 unsigned LLVMGetNumMaskElements(LLVMValueRef ShuffleVectorInst);
 
+/**
+ * \returns a constant that specifies that the result of a \c ShuffleVectorInst
+ * is undefined.
+ */
+int LLVMGetUndefMaskElem(void);
+
 /**
  * Get the mask value at position Elt in the mask of a ShuffleVector
- * instruction. Return LLVMUndefMaskElem if the mask value is undef at that
- * position.
+ * instruction.
+ *
+ * \Returns the result of \c LLVMGetUndefMaskElem() if the mask value is undef
+ * at that position.
  */
 int LLVMGetMaskValue(LLVMValueRef ShuffleVectorInst, unsigned Elt);
-extern const int LLVMUndefMaskElem;
 
 LLVMBool LLVMIsAtomicSingleThread(LLVMValueRef AtomicInst);
 void LLVMSetAtomicSingleThread(LLVMValueRef AtomicInst, LLVMBool SingleThread);

llvm/lib/IR/Core.cpp

@@ -3963,9 +3963,8 @@ int LLVMGetMaskValue(LLVMValueRef SVInst, unsigned Elt) {
   ShuffleVectorInst *I = cast<ShuffleVectorInst>(P);
   return I->getMaskValue(Elt);
 }
-const int LLVMUndefMaskElem =
-    -1; // not actually accessible as ShuffleVectorInst::UndefMaskElem, so we
-        // hardcode it here
+
+int LLVMGetUndefMaskElem(void) { return UndefMaskElem; }
 
 LLVMBool LLVMIsAtomicSingleThread(LLVMValueRef AtomicInst) {
   Value *P = unwrap<Value>(AtomicInst);

llvm/tools/llvm-c-test/echo.cpp

@@ -809,7 +809,7 @@ struct FunCloner {
         unsigned NumMaskElts = LLVMGetNumMaskElements(Src);
         for (unsigned i = 0; i < NumMaskElts; i++) {
           int Val = LLVMGetMaskValue(Src, i);
-          if (Val == LLVMUndefMaskElem) {
+          if (Val == LLVMGetUndefMaskElem()) {
             MaskElts.push_back(LLVMGetUndef(LLVMInt64Type()));
           } else {
             MaskElts.push_back(LLVMConstInt(LLVMInt64Type(), Val, true));