[llvm] SystemZ: Stop casting fp typed atomic loads in the IR (PR #90768)

[Ulrich Weigand via llvm-commits]

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@@ -6249,6 +6249,22 @@ SDValue SystemZTargetLowering::LowerOperation(SDValue Op,
   }
 }
 
+// Manually lower a bitcast to avoid introducing illegal types after type
+// legalization.
+static SDValue expandBitCastI128ToF128(SelectionDAG &DAG, SDValue Src,
+                                       SDValue Chain, const SDLoc &SL) {
+  SDValue Hi =
+      DAG.getTargetExtractSubreg(SystemZ::subreg_h64, SL, MVT::i64, Src);
+  SDValue Lo =
+      DAG.getTargetExtractSubreg(SystemZ::subreg_l64, SL, MVT::i64, Src);
+
+  Hi = DAG.getBitcast(MVT::f64, Hi);
+  Lo = DAG.getBitcast(MVT::f64, Lo);
+
+  SDNode *Pair = DAG.getMachineNode(SystemZ::PAIR128, SL, MVT::f128, Hi, Lo);
----------------
uweigand wrote:

I'm assuming this pre-reqs https://github.com/llvm/llvm-project/pull/90640 (otherwise emitPair128 would hard-code GR128 temporaries)?   However, even so PAIR128 still hard-codes GR128 here:
```
  def PAIR128 : Pseudo<(outs GR128:$dst), (ins GR64:$hi, GR64:$lo), []>;
```
so this presumably would fail with the verifier?

https://github.com/llvm/llvm-project/pull/90768