[RISCV] Lower fixed-length vp_{gather,scatter} for zvfhmin/zvfbfmin (#115253)

This uses the same lowering as masked gathers and scatters.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -1339,12 +1339,11 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
                             ISD::VECTOR_SHUFFLE, ISD::VECTOR_COMPRESS},
                            VT, Custom);
 
-        // FIXME: vp_gather/scatter can be hoisted to here.
         setOperationAction({ISD::LOAD, ISD::STORE, ISD::MLOAD, ISD::MSTORE,
                             ISD::MGATHER, ISD::MSCATTER},
                            VT, Custom);
-        setOperationAction({ISD::VP_LOAD, ISD::VP_STORE,
-                            ISD::EXPERIMENTAL_VP_STRIDED_LOAD,
+        setOperationAction({ISD::VP_LOAD, ISD::VP_STORE, ISD::VP_GATHER,
+                            ISD::VP_SCATTER, ISD::EXPERIMENTAL_VP_STRIDED_LOAD,
                             ISD::EXPERIMENTAL_VP_STRIDED_STORE},
                            VT, Custom);
 
@@ -1409,8 +1408,6 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
         setOperationAction({ISD::BUILD_VECTOR, ISD::SCALAR_TO_VECTOR}, VT,
                            Custom);
 
-        setOperationAction({ISD::VP_GATHER, ISD::VP_SCATTER}, VT, Custom);
-
         setOperationAction({ISD::FADD, ISD::FSUB, ISD::FMUL, ISD::FDIV,
                             ISD::FNEG, ISD::FABS, ISD::FCOPYSIGN, ISD::FSQRT,
                             ISD::FMA, ISD::FMINNUM, ISD::FMAXNUM,