[llvm] [RISCV] Don't run combineBinOp_VLToVWBinOp_VL until after legalize types. NFCI (PR #84125)

[Philip Reames via llvm-commits]

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@@ -13652,10 +13652,6 @@ struct NodeExtensionHelper {
       unsigned ScalarBits = VT.getScalarSizeInBits();
       unsigned NarrowScalarBits = NarrowVT.getScalarSizeInBits();
 
-      // Ensure the narrowing element type is legal
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preames wrote:

Can you do me a favor and concern this check into an assertion?  That way it's locally obvious that the type must be legal when this bit of code runs.  This mostly matters in your follow up review.

https://github.com/llvm/llvm-project/pull/84125