[llvm] [RISCV] Handle scalable ops with < EEW / 2 narrow types in combineBinOp_VLToVWBinOp_VL (PR #84158)

[Luke Lau via llvm-commits]

================
@@ -13646,20 +13646,6 @@ struct NodeExtensionHelper {
       if (!VT.isVector())
         break;
 
-      SDValue NarrowElt = OrigOperand.getOperand(0);
-      MVT NarrowVT = NarrowElt.getSimpleValueType();
-
-      unsigned ScalarBits = VT.getScalarSizeInBits();
-      unsigned NarrowScalarBits = NarrowVT.getScalarSizeInBits();
-
-      // Ensure the narrowing element type is legal
-      if (!Subtarget.getTargetLowering()->isTypeLegal(NarrowElt.getValueType()))
-        break;
-
-      // Ensure the extension's semantic is equivalent to rvv vzext or vsext.
-      if (ScalarBits != NarrowScalarBits * 2)
----------------
lukel97 wrote:

> If after the prior change which moves this transform after legalize types, the only case which needs this restriction to keep the transform between legalize types and legalize ops is the i1 vector case, why not simply check if the narrow vt is a i1 vector here?

I moved it to after the legalize vector ops phase since we weren't checking for i1 vectors in any of the other _VL nodes. So I think there was already an implicit invariant here that the combine would only run after legalize ops, and it seemed safer to just be explicit about it. 

> Wouldn't that be less disruptive than moving the combine after legalize ops?

Since the combine was already happening after legalize ops for the _VL nodes, this should only affect the ISD::ADD/SUB/MUL nodes that were added in #76785

https://github.com/llvm/llvm-project/pull/84158