[llvm] [DAGCombiner] Fold min/max vscale, C -> C (PR #174708)

[Luke Lau via llvm-commits]

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@@ -6253,6 +6243,34 @@ SDValue DAGCombiner::visitIMINMAX(SDNode *N) {
                                         SDLoc(N), VT, N0, N1))
     return SD;
 
+  // Fold operation with vscale operands.
+  if (N0.getOpcode() == ISD::VSCALE && N1.getOpcode() == ISD::VSCALE) {
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lukel97 wrote:

Also separately, I ran into the issue where I think the vscale SelectionDAG node can wrap with its scale operand. I.e if we don't apply vscale_range in the tests, this combine doesn't kick in because the vscale node can overflow to zero. 

This vscale is generated from a TypeSize which IIUC can't overflow 64 bits? Maybe we need a nuw flag for vscale so we don't lose this information, or instead explicitly expand it out to a `mul nuw N, (vscale 1)`

https://github.com/llvm/llvm-project/pull/174708