[llvm-exegesis] Allow retries on flaky middle half test

The middle half repetition mode test sometimes fails on the avx512
buildbots due to a negative value being produced. This needs more
investigation, but add a retry count temporarily to alleviate
false-positive errors on the buildbots.

This is being tracked in llvm#80957.

llvm/test/tools/llvm-exegesis/X86/latency/middle-half.s

@@ -1,5 +1,8 @@
 # REQUIRES: exegesis-can-measure-latency, x86_64-linux
 
+# TODO(boomanaiden154): This sometimes fails with a negative latency value.
+# ALLOW_RETRIES: 2
+
 # Check that we can use the middle-half repetition mode without crashing
 
 # RUN: llvm-exegesis -mtriple=x86_64-unknown-unknown -mode=latency -opcode-name=ADD64rr -repetition-mode=middle-half-duplicate | FileCheck %s