[llvm] [SLP][REVEC] Initial commits. (PR #98269)

[Han-Kuan Chen via llvm-commits]

================
@@ -227,13 +231,24 @@ static const unsigned MaxPHINumOperands = 128;
 /// avoids spending time checking the cost model and realizing that they will
 /// be inevitably scalarized.
 static bool isValidElementType(Type *Ty) {
+  // TODO: Support ScalableVectorType.
----------------
HanKuanChen wrote:

Add assert in `isValidElementType` will make SLP cannot work. But we can add assert inside `getNumElements`.

https://github.com/llvm/llvm-project/pull/98269