[BOLT][test] Fix NFC check with pre-aggregated-perf.test (llvm#113944)

NFC checks have been failing starting with
https://lab.llvm.org/buildbot/#/builders/92/builds/8567.

NFC testing wrapper (llvm-bolt-wrapper) replaces the call of `perf2bolt`
with `llvm-bolt --aggregate-only --ignore-build-id`.

`show-density` is automatically enabled for perf2bolt only but not for
`llvm-bolt --aggregate-only`. Add the flag to the test to work around
the issue.

Test Plan:
```
cd build
../llvm-project/bolt/utils/nfc-check-setup.py --switch-back --verbose
bin/llvm-lit -a tools/bolt/test/X86/pre-aggregated-perf.test
```

bolt/test/X86/pre-aggregated-perf.test

@@ -11,13 +11,15 @@ REQUIRES: system-linux
 
 RUN: yaml2obj %p/Inputs/blarge.yaml &> %t.exe
 RUN: perf2bolt %t.exe -o %t --pa -p %p/Inputs/pre-aggregated.txt -w %t.new \
+RUN:   --show-density \
 RUN:   --profile-density-threshold=9 --profile-density-cutoff-hot=970000 \
 RUN:   --profile-use-dfs | FileCheck %s --check-prefix=CHECK-P2B
 
 CHECK-P2B: BOLT-INFO: 4 out of 7 functions in the binary (57.1%) have non-empty execution profile
 CHECK-P2B: BOLT-INFO: Functions with density >= 21.7 account for 97.00% total sample counts.
 
 RUN: perf2bolt %t.exe -o %t --pa -p %p/Inputs/pre-aggregated.txt -w %t.new \
+RUN:   --show-density \
 RUN:   --profile-density-cutoff-hot=970000 \
 RUN:   --profile-use-dfs 2>&1 | FileCheck %s --check-prefix=CHECK-WARNING