Enable customer lowering for fabs_v16f16 with AVX2 (llvm#72914)

This is part-2 change to improve codegen for vec_fabs. In this patch,
v16f16 and v132f16 fabs are improved.

There will be at least two followups patches after this one.
1) fixing the ISEL crash when fabs.v32f16 uses custom lowering with
AVX512
2) better expansion for v16f16, v32f16 types on AVX1 subtargets.

Author: david-xl

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -1596,6 +1596,8 @@ X86TargetLowering::X86TargetLowering(const X86TargetMachine &TM,
       setOperationAction(ISD::STORE,              VT, Custom);
     }
     setF16Action(MVT::v16f16, Expand);
+    if (Subtarget.hasAVX2())
+      setOperationAction(ISD::FABS, MVT::v16f16, Custom);
     setOperationAction(ISD::FADD, MVT::v16f16, Expand);
     setOperationAction(ISD::FSUB, MVT::v16f16, Expand);
     setOperationAction(ISD::FMUL, MVT::v16f16, Expand);