[llvm] [AMDGPU][SDAG] Legalise v2i32 or/xor/and instructions to make use of 64-bit wide instructions (PR #140694)

[Janek van Oirschot via llvm-commits]

================
@@ -3225,29 +3225,51 @@ bool AMDGPUDAGToDAGISel::SelectVOP3ModsImpl(SDValue In, SDValue &Src,
   if (IsCanonicalizing)
     return true;
 
-  unsigned Opc = Src->getOpcode();
+  // v2i32 xor/or/and are legal. A vselect using these instructions as operands
+  // is scalarised into two selects with EXTRACT_VECTOR_ELT operands. Peek
+  // through the extract to the bitwise op.
+  SDValue PeekSrc =
+      Src->getOpcode() == ISD::EXTRACT_VECTOR_ELT ? Src->getOperand(0) : Src;
+  // Convert various sign-bit masks to src mods. Currently disabled for 16-bit
+  // types as the codegen replaces the operand without adding a srcmod.
+  // This is intentionally finding the cases where we are performing float neg
+  // and abs on int types, the goal is not to obtain two's complement neg or
+  // abs.
+  // TODO: Add 16-bit support.
+  unsigned Opc = PeekSrc.getOpcode();
   EVT VT = Src.getValueType();
   if ((Opc != ISD::AND && Opc != ISD::OR && Opc != ISD::XOR) ||
-      (VT != MVT::i32 && VT != MVT::i64))
+      (VT != MVT::i32 && VT != MVT::v2i32 && VT != MVT::i64))
     return true;
 
-  ConstantSDNode *CRHS = dyn_cast<ConstantSDNode>(Src->getOperand(1));
+  ConstantSDNode *CRHS = isConstOrConstSplat(PeekSrc ? PeekSrc->getOperand(1)
+                                                     : Src->getOperand(1));
   if (!CRHS)
     return true;
 
+  auto ReplaceSrc = [&]() -> SDValue {
+    if (Src->getOpcode() != ISD::EXTRACT_VECTOR_ELT)
+      return PeekSrc.getOperand(0);
----------------
JanekvO wrote:

```suggestion
      return Src.getOperand(0);
```
Just my $0.02 but: I know `PeekSrc == Src` given the if statement and ternary assignment of `PeekSrc`, but now it looks like there's an explicit reason as to why `PeekSrc.getOperand(0)` was used instead of the `Src` equivalent.

https://github.com/llvm/llvm-project/pull/140694