[LLVM][AArch64] Change SVE CodeGen tests to use splat().

The affected tests were using the longwinded syntax for constant
splats. By using the splat() syntax the tests get simplified whilst
also removing the need for "undef".

Author: paulwalker-arm

llvm/test/CodeGen/AArch64/pr49781.ll

@@ -6,8 +6,7 @@ define <vscale x 2 x i64> @foo(<vscale x 2 x i64> %a) {
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    sub z0.d, z0.d, #2 // =0x2
 ; CHECK-NEXT:    ret
- %idx = shufflevector <vscale x 2 x i64> insertelement (<vscale x 2 x i64> undef, i64 1, i32 0), <vscale x 2 x i64> zeroinitializer, <vscale x 2 x i32> zeroinitializer
- %b = sub <vscale x 2 x i64> %a, %idx
- %c = sub <vscale x 2 x i64> %b, %idx
+ %b = sub <vscale x 2 x i64> %a, splat (i64 1)
+ %c = sub <vscale x 2 x i64> %b, splat (i64 1)
  ret <vscale x 2 x i64> %c
 }