[llvm] [RISCV] Don't create BuildPairF64 or SplitF64 nodes without D or Zdinx. (PR #116159)

[Sam Elliott via llvm-commits]

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@@ -6422,7 +6423,8 @@ SDValue RISCVTargetLowering::LowerOperation(SDValue Op,
       SDValue NewOp0 = DAG.getNode(ISD::ANY_EXTEND, DL, MVT::i64, Op0);
       return DAG.getNode(RISCVISD::FMV_W_X_RV64, DL, MVT::f32, NewOp0);
     }
-    if (VT == MVT::f64 && Op0VT == MVT::i64 && XLenVT == MVT::i32) {
+    if (VT == MVT::f64 && Op0VT == MVT::i64 && XLenVT == MVT::i32 &&
+        Subtarget.hasStdExtDOrZdinx()) {
----------------
lenary wrote:

Just a readability suggestion:

```suggestion
    if (VT == MVT::f64 && Op0VT == MVT::i64 && !Subtarget.is64Bit() &&
        Subtarget.hasStdExtDOrZdinx()) {
```

Writing the conditions like this makes them match a lot more closely to the conditions on the relevant `setOperationAction(ISD::BITCAST, MVT::i64, Custom);` (on line 555 I believe)

I'll remove my similar change from the GPR Pairs PR, as it's no longer really relevant there.

https://github.com/llvm/llvm-project/pull/116159