[Docs] Update llvm-test-suite PGO instructions to use LLVM IR PGO by default

[mingmingl]

Summary

• Currently, the default instructions will use Clang's frontend PGO feature.

[mtrofin]

llvm/docs/TestSuiteGuide.md

355

I would suggest either making TEST_SUITE_USE_IR_PGO=On default, or at minimum setting it to On in this example (e.g. line 340) and then flipping the comment on line 354.

[mtrofin]

LGTM, but I'd wait to see if anyone has a strong dependency on the flag being the other way around.

[aeubanks]

llvm/docs/TestSuiteGuide.md

355

+1, just change the default in llvm-test-suite and remove -DTEST_SUITE_USE_IR_PGO=ON from the docs

[mingmingl]

llvm/docs/TestSuiteGuide.md

355

Thanks for the input!

Given that front-end instrumentation (i.e., fprofile-instr-generate) is mentioned in user-manual (https://clang.llvm.org/docs/UsersManual.html#differences-between-sampling-and-instrumentation) and option-handling, I'm leaning towards keeping the default in case people rely on it in the daily testing workflow.

[aeubanks]

llvm/docs/TestSuiteGuide.md

355

I thought we were trying to move away from -fprofile-instr-generate for any sort of PGO and only use it for code coverage, but that requires documentation updating

llvm/docs/TestSuiteGuide.md

 
 Profile guided optimization requires to compile and run twice. First the
 benchmark should be compiled with profile generation instrumentation enabled
 and setup for training data. The lit runner will merge the profile files
 using `llvm-profdata` so they can be used by the second compilation run.
 
 Example:
 ```bash
-# Profile generation run:
+# Profile generation run using LLVM IR PGO:
 % cmake -DTEST_SUITE_PROFILE_GENERATE=ON \
+        -DTEST_SUITE_USE_IR_PGO=ON \
         -DTEST_SUITE_RUN_TYPE=train \
         ../test-suite
 % make
 % llvm-lit .
 # Use the profile data for compilation and actual benchmark run:
 % cmake -DTEST_SUITE_PROFILE_GENERATE=OFF \
         -DTEST_SUITE_PROFILE_USE=ON \
         -DTEST_SUITE_RUN_TYPE=ref \
         .
 % make
 % llvm-lit -o result.json .
 ```
 
+To use Clang frontend's PGO instead of LLVM IR PGO, set `-DTEST_SUITE_USE_IR_PGO=OFF`.
+
 The `TEST_SUITE_RUN_TYPE` setting only affects the SPEC benchmark suites.
 
 
 Cross Compilation and External Devices
 --------------------------------------
 
 ### Compilation