[SVE] Add checks for no warnings in CodeGen/AArch64/sve-sext-zext.ll

[david-arm]

Summary

Previous patches fixed up all the warnings in this test:

llvm/test/CodeGen/AArch64/sve-sext-zext.ll

and this change simply checks that no new warnings are added in future.

[sdesmalen]

LGTM

llvm/test/CodeGen/AArch64/sve-sext-zext.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -mtriple=aarch64--linux-gnu -mattr=+sve < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64--linux-gnu -mattr=+sve < %s 2>%t | FileCheck %s
+; RUN: FileCheck --check-prefix=WARN --allow-empty %s <%t
+
+; WARN-NOT: warning
 
 define <vscale x 16 x i8> @sext_i1_i8(<vscale x 16 x i1> %a) {
 ; CHECK-LABEL: sext_i1_i8:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    mov z0.b, p0/z, #-1 // =0xffffffffffffffff
 ; CHECK-NEXT:    ret
   %r = sext <vscale x 16 x i1> %a to <vscale x 16 x i8>
   ret <vscale x 16 x i8> %r