[RISCV] Fix incorrect mask of shuffle vector in the test. (NFC) (#130244)

The mask of shuffle vector should be <u, u, 4, 6, 8, 10, 12, 14>, not
<u, u, 4, 6, *6, 10, 12, 14> for steps of 2.

And the mask of suffle vector with an undef initial element has been
supported by #118509.

Author: tclin914

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-shufflevector-vnsrl.ll

@@ -380,30 +380,18 @@ entry:
   ret void
 }
 
-; TODO: Allow an undef initial element
 define void @vnsrl_0_i8_undef3(ptr %in, ptr %out) {
 ; CHECK-LABEL: vnsrl_0_i8_undef3:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    vsetivli zero, 16, e8, mf2, ta, ma
 ; CHECK-NEXT:    vle8.v v8, (a0)
-; CHECK-NEXT:    vmv.v.i v0, 8
-; CHECK-NEXT:    vsetivli zero, 8, e8, mf4, ta, mu
-; CHECK-NEXT:    vid.v v9
-; CHECK-NEXT:    li a0, -32
-; CHECK-NEXT:    vadd.vv v9, v9, v9
-; CHECK-NEXT:    vadd.vi v9, v9, -8
-; CHECK-NEXT:    vslidedown.vi v10, v8, 2
-; CHECK-NEXT:    vslidedown.vi v10, v8, 3, v0.t
-; CHECK-NEXT:    vmv.s.x v0, a0
-; CHECK-NEXT:    vsetivli zero, 8, e8, mf2, ta, ma
-; CHECK-NEXT:    vslidedown.vi v8, v8, 8
-; CHECK-NEXT:    vsetivli zero, 8, e8, mf4, ta, mu
-; CHECK-NEXT:    vrgather.vv v10, v8, v9, v0.t
-; CHECK-NEXT:    vse8.v v10, (a1)
+; CHECK-NEXT:    vsetivli zero, 8, e8, mf4, ta, ma
+; CHECK-NEXT:    vnsrl.wi v8, v8, 0
+; CHECK-NEXT:    vse8.v v8, (a1)
 ; CHECK-NEXT:    ret
 entry:
   %0 = load <16 x i8>, ptr %in, align 1
-  %shuffle.i5 = shufflevector <16 x i8> %0, <16 x i8> poison, <8 x i32> <i32 undef, i32 undef, i32 4, i32 6, i32 6, i32 10, i32 12, i32 14>
+  %shuffle.i5 = shufflevector <16 x i8> %0, <16 x i8> poison, <8 x i32> <i32 undef, i32 undef, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>
   store <8 x i8> %shuffle.i5, ptr %out, align 1
   ret void
 }