[LV][NFC] Add "REQUIRES: assert" to new test file (#180522)

Fixes a minor test regression introduced by
https://github.com/llvm/llvm-project/pull/180226 in file
llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll

llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll

diff --git a/llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll b/llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll
index f685bf8..e451f94 100644
--- a/llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll
+++ b/llvm/test/Transforms/LoopVectorize/phi-with-fastflags-vplan.ll
@@ -1,4 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 6
+; REQUIRES: asserts
 ; RUN: opt -p loop-vectorize -force-vector-width=4 -debug-only=loop-vectorize -disable-output -S %s 2>&1 | FileCheck %s
 
 define void @f(ptr noalias %p, i1 %c) {