[RISCV] Add test for strided gather with recursive disjoint or. NFC

This already gets converted to a strided intrinsic because we currently call
haveNoCommonBitsSet when checking or instructions, but an upcoming patch will
change this logic and we want to preserve this case.

Note that this IR is in the form that comes from instcombine. The splats need
to be inline constexprs, otherwise isSplatValue() will fail. (It can't
currently handle splats where the shufflevector is an instruction, and the
insertelement is a constexpr.

Author: lukel97

llvm/test/CodeGen/RISCV/rvv/strided-load-store.ll

@@ -49,6 +49,58 @@ for.cond.cleanup:                                 ; preds = %vector.body
   ret <vscale x 1 x i64> %accum.next
 }
 
+define <vscale x 1 x i64> @gather_disjoint_or(ptr %a, i64 %len) {
+; CHECK-LABEL: @gather_disjoint_or(
+; CHECK-NEXT:  vector.ph:
+; CHECK-NEXT:    [[VSCALE:%.*]] = call i64 @llvm.vscale.i64()
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH:%.*]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_IND_SCALAR:%.*]] = phi i64 [ 1, [[VECTOR_PH]] ], [ [[VEC_IND_NEXT_SCALAR:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[ACCUM:%.*]] = phi <vscale x 1 x i64> [ zeroinitializer, [[VECTOR_PH]] ], [ [[ACCUM_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i64, ptr [[A:%.*]], i64 [[VEC_IND_SCALAR]]
+; CHECK-NEXT:    [[GATHER:%.*]] = call <vscale x 1 x i64> @llvm.riscv.masked.strided.load.nxv1i64.p0.i64(<vscale x 1 x i64> poison, ptr [[TMP0]], i64 16, <vscale x 1 x i1> shufflevector (<vscale x 1 x i1> insertelement (<vscale x 1 x i1> poison, i1 true, i32 0), <vscale x 1 x i1> poison, <vscale x 1 x i32> zeroinitializer))
+; CHECK-NEXT:    [[ACCUM_NEXT]] = add <vscale x 1 x i64> [[ACCUM]], [[GATHER]]
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], [[VSCALE]]
+; CHECK-NEXT:    [[VEC_IND_NEXT_SCALAR]] = add i64 [[VEC_IND_SCALAR]], 2
+; CHECK-NEXT:    [[EXIT:%.*]] = icmp ne i64 [[INDEX_NEXT]], [[LEN:%.*]]
+; CHECK-NEXT:    br i1 [[EXIT]], label [[FOR_COND_CLEANUP:%.*]], label [[VECTOR_BODY]]
+; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    ret <vscale x 1 x i64> [[ACCUM_NEXT]]
+;
+vector.ph:
+  %vscale = call i64 @llvm.vscale.i64()
+  %step = tail call <vscale x 1 x i64> @llvm.experimental.stepvector.nxv1i64()
+  %step.mul2 = shl <vscale x 1 x i64> %step, shufflevector(<vscale x 1 x i64> insertelement(<vscale x 1 x i64> poison, i64 1, i32 0), <vscale x 1 x i64> poison, <vscale x 1 x i32> zeroinitializer)
+  br label %vector.body
+
+vector.body:                                      ; preds = %vector.body, %vector.ph
+  %index = phi i64 [ 0, %vector.ph ], [ %index.next, %vector.body ]
+  %vec.ind = phi <vscale x 1 x i64> [ %step.mul2, %vector.ph ], [ %vec.ind.next, %vector.body ]
+
+  %accum = phi <vscale x 1 x i64> [ zeroinitializer, %vector.ph ], [ %accum.next, %vector.body ]
+
+  %vec.ind.or = or disjoint <vscale x 1 x i64> %vec.ind, shufflevector(<vscale x 1 x i64> insertelement(<vscale x 1 x i64> poison, i64 1, i64 0), <vscale x 1 x i64> poison, <vscale x 1 x i32> zeroinitializer)
+
+  %gep = getelementptr i64, ptr %a, <vscale x 1 x i64> %vec.ind.or
+  %gather = call <vscale x 1 x i64> @llvm.masked.gather.nxv1i64.nxv1p0(
+  <vscale x 1 x ptr> %gep,
+  i32 8,
+  <vscale x 1 x i1> shufflevector (<vscale x 1 x i1> insertelement (<vscale x 1 x i1> poison, i1 true, i32 0), <vscale x 1 x i1> poison, <vscale x 1 x i32> zeroinitializer),
+  <vscale x 1 x i64> poison
+  )
+
+  %accum.next = add <vscale x 1 x i64> %accum, %gather
+  %index.next = add nuw i64 %index, %vscale
+  %vec.ind.next = add <vscale x 1 x i64> %vec.ind, shufflevector(<vscale x 1 x i64> insertelement(<vscale x 1 x i64> poison, i64 2, i64 0), <vscale x 1 x i64> poison, <vscale x 1 x i32> zeroinitializer)
+
+  %exit = icmp ne i64 %index.next, %len
+  br i1 %exit, label %for.cond.cleanup, label %vector.body
+
+for.cond.cleanup:                                 ; preds = %vector.body
+  ret <vscale x 1 x i64> %accum.next
+}
+
 define void @scatter(ptr %a, i32 %len) {
 ; CHECK-LABEL: @scatter(
 ; CHECK-NEXT:  vector.ph: