[LLVM][IR] Use splat syntax when printing Constant[Data]Vector.

Author: paulwalker-arm

llvm/lib/IR/AsmWriter.cpp

@@ -1690,6 +1690,23 @@ static void WriteConstantInternal(raw_ostream &Out, const Constant *CV,
   if (isa<ConstantVector>(CV) || isa<ConstantDataVector>(CV)) {
     auto *CVVTy = cast<FixedVectorType>(CV->getType());
     Type *ETy = CVVTy->getElementType();
+
+    // Use the same shorthand for splat vector (i.e. "splat(Ty val)") as is
+    // permitted on IR input to reduce the output changes when enabling
+    // UseConstant{Int,FP}ForFixedLengthSplat.
+    // TODO: Remove this block when the UseConstant{Int,FP}ForFixedLengthSplat
+    // options are removed.
+    if (auto *SplatVal = CV->getSplatValue()) {
+      if (isa<ConstantInt>(SplatVal) || isa<ConstantFP>(SplatVal)) {
+        Out << "splat (";
+        WriterCtx.TypePrinter->print(ETy, Out);
+        Out << " ";
+        WriteAsOperandInternal(Out, SplatVal, WriterCtx);
+        Out << ")";
+        return;
+      }
+    }
+
     Out << '<';
     WriterCtx.TypePrinter->print(ETy, Out);
     Out << ' ';