[llvm] [X86] Enhance FABS/FNEG lowering for scalar _Float16 with bitwise operations (PR #128637)

[Simon Pilgrim via llvm-commits]

================
@@ -22313,7 +22344,7 @@ static SDValue LowerFABSorFNEG(SDValue Op, SelectionDAG &DAG) {
   // generate a 16-byte vector constant and logic op even for the scalar case.
   // Using a 16-byte mask allows folding the load of the mask with
   // the logic op, so it can save (~4 bytes) on code size.
-  bool IsFakeVector = !VT.isVector() && !IsF128;
+  bool IsFakeVector = !VT.isVector() && !IsF128 && VT != MVT::f16;
----------------
RKSimon wrote:

We make the MVT::f16 type legal for all SSE2+ targets:
```c
    addRegisterClass(MVT::f16, Subtarget.hasAVX512() ? &X86::FR16XRegClass
                                                     : &X86::FR16RegClass);
```
Later on we setup default actions for all the F16 opcodes with the setF16Action helper (including FNEG/FABS/FCOPYSIGN), and then override some of them:
```c
    setF16Action(MVT::f16, Promote);
    setOperationAction(ISD::FADD, MVT::f16, Promote);
    setOperationAction(ISD::FSUB, MVT::f16, Promote);
    setOperationAction(ISD::FMUL, MVT::f16, Promote);
    setOperationAction(ISD::FDIV, MVT::f16, Promote);
    setOperationAction(ISD::FP_ROUND, MVT::f16, Custom);
    setOperationAction(ISD::FP_EXTEND, MVT::f32, Custom);
    setOperationAction(ISD::FP_EXTEND, MVT::f64, Custom);
```
For MVT::v8f16 we do this again, this time remembering to reset FNEG/FABS/FCOPYSIGN to Custom:
```c
    setF16Action(MVT::v8f16, Expand);
    setOperationAction(ISD::FADD, MVT::v8f16, Expand);
    setOperationAction(ISD::FSUB, MVT::v8f16, Expand);
    setOperationAction(ISD::FMUL, MVT::v8f16, Expand);
    setOperationAction(ISD::FDIV, MVT::v8f16, Expand);
    setOperationAction(ISD::FNEG, MVT::v8f16, Custom);
    setOperationAction(ISD::FABS, MVT::v8f16, Custom);
    setOperationAction(ISD::FCOPYSIGN, MVT::v8f16, Custom);
```
We should have done something similar for the MVT::f16 cases earlier.

https://github.com/llvm/llvm-project/pull/128637