[AArch64][SVE2] Use rshrnb for masked stores

This patch is a follow up on https://reviews.llvm.org/D155299.
This patch combines add+lsr to rshrnb when 'B' in:

  C = A + B
  D = C >> Shift

is equal to (1 << (Shift-1), and the bits in the top half
of each vector element are zeroed or ignored, such as in a
truncating masked store.

Author: MDevereau

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -21017,6 +21017,21 @@ static SDValue performMSTORECombine(SDNode *N,
     }
   }
 
+  if (MST->isTruncatingStore()) {
+    if (SDValue Rshrnb = trySimplifySrlAddToRshrnb(Value, DAG, Subtarget)) {
+      EVT ValueVT = Value->getValueType(0);
+      EVT MemVT = MST->getMemoryVT();
+      if ((ValueVT == MVT::nxv8i16 && MemVT == MVT::nxv8i8) ||
+          (ValueVT == MVT::nxv4i32 && MemVT == MVT::nxv4i16) ||
+          (ValueVT == MVT::nxv2i64 && MemVT == MVT::nxv2i32)) {
+        return DAG.getMaskedStore(
+            MST->getChain(), DL, Rshrnb, MST->getBasePtr(), MST->getOffset(),
+            MST->getMask(), MST->getMemoryVT(), MST->getMemOperand(),
+            MST->getAddressingMode(), true);
+      }
+    }
+  }
+
   return SDValue();
 }
 

llvm/test/CodeGen/AArch64/sve2-intrinsics-combine-rshrnb.ll

@@ -298,3 +298,22 @@ define void @neg_add_lshr_rshrnb_s(ptr %ptr, ptr %dst, i64 %index){
   store <vscale x 2 x i16> %3, ptr %4, align 1
   ret void
 }
+
+define void @masked_store_rshrnb(ptr %ptr, ptr %dst, i64 %index, <vscale x 8 x i1> %mask) {                             ; preds = %vector.body, %vector.ph
+; CHECK-LABEL: masked_store_rshrnb:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ld1h { z0.h }, p0/z, [x0]
+; CHECK-NEXT:    rshrnb z0.b, z0.h, #6
+; CHECK-NEXT:    st1b { z0.h }, p0, [x1, x2]
+; CHECK-NEXT:    ret
+  %wide.masked.load = tail call <vscale x 8 x i16> @llvm.masked.load.nxv8i16.p0(ptr %ptr, i32 2, <vscale x 8 x i1> %mask, <vscale x 8 x i16> poison)
+  %1 = add <vscale x 8 x i16> %wide.masked.load, trunc (<vscale x 8 x i32> shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 32, i64 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer) to <vscale x 8 x i16>)
+  %2 = lshr <vscale x 8 x i16> %1, trunc (<vscale x 8 x i32> shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 6, i64 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer) to <vscale x 8 x i16>)
+  %3 = trunc <vscale x 8 x i16> %2 to <vscale x 8 x i8>
+  %4 = getelementptr inbounds i8, ptr %dst, i64 %index
+  tail call void @llvm.masked.store.nxv8i8.p0(<vscale x 8 x i8> %3, ptr %4, i32 1, <vscale x 8 x i1> %mask)
+  ret void
+}
+
+declare void @llvm.masked.store.nxv8i8.p0(<vscale x 8 x i8>, ptr, i32, <vscale x 8 x i1>)
+declare <vscale x 8 x i16> @llvm.masked.load.nxv8i16.p0(ptr, i32, <vscale x 8 x i1>, <vscale x 8 x i16>)