[clang] [llvm] [LLVM][IR] Use splat syntax when printing Constant[Data]Vector. (PR #112548)

[Paul Walker via cfe-commits]

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@@ -1687,6 +1692,46 @@ static void WriteConstantInternal(raw_ostream &Out, const Constant *CV,
     return;
   }
 
+  // When in the mode where Constant{Int,FP} do not support vector types the
+  // "splat(Ty val)" syntax is interpreted as a ConstantDataVector. Maintaining
+  // this association when outputiing the IR will significantly reduce the
+  // output changes when in the mode where Constant{Int,FP} do support vector
+  // types. In turn this should make it easier to spot difference in output
+  // when switching between the modes. Once the transition is complete this
+  // code will be removed.
+  if (const ConstantDataVector *CDV = dyn_cast<ConstantDataVector>(CV)) {
+    if (auto *SplatVal = CDV->getSplatValue()) {
+      Type* EltTy = SplatVal->getType();
+
+      if (EltTy->isIntegerTy() && !UseConstantIntForFixedLengthSplat) {
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paulwalker-arm wrote:

As predicted the conditions did not affect the behaviour and have been removed.

The one place where constants are still printed as `<Ty V1, Ty V2...>` is shufflevector masks, which go down a dedicated path.  I've kept this as is for now because figure non-zero splat masks are rare and `ShuffleVectorInst::isValidOperands` currently rejects `ConstantInt` anyway.

https://github.com/llvm/llvm-project/pull/112548