[llvm] [InstCombine] Add contributor guide (PR #79007)

[Yingwei Zheng via llvm-commits]

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+# InstCombine contributor guide
+
+This guide lays out a series of rules that contributions to InstCombine should
+follow. **Following these rules will results in much faster PR approvals.**
+
+## Tests
+
+### Precommit tests
+
+Tests for new optimizations or miscompilation fixes should be pre-committed.
+This means that you first commit the test with CHECK lines showing the behavior
+*without* your change. Your actual change will then only contain CHECK line
+diffs relative to that baseline.
+
+This means that pull requests should generally contain two commits: First,
+one commit adding new tests with baseline check lines. Second, a commit with
+functional changes and test diffs.
+
+If the second commit in your PR does not contain test diffs, you did something
+wrong. Either you made a mistake when generating CHECK lines, or your tests are
+not actually affected by your patch.
+
+Exceptions: When fixing assertion failures or infinite loops, do not pre-commit
+tests.
+
+### Use `update_test_checks.py`
+
+CHECK lines should be generated using the `update_test_checks.py` script. Do
+**not** manually edit check lines after using it.
+
+Be sure to use the correct opt binary when using the script. For example, if
+your build directory is `build`, then you'll want to run:
+
+```sh
+llvm/utils/update_test_checks.py --opt-binary build/bin/opt \
+    llvm/test/Transforms/InstCombine/the_test.ll
+```
+
+Exceptions: Hand-written CHECK lines are allowed for debuginfo tests.
+
+### General testing considerations
+
+Place all tests relating to a transform into a single file. If you are adding
+a regression test for a crash/miscompile in an existing transform, find the
+file where the existing tests are located. A good way to do that is to comment
+out the transform and see which tests fail.
+
+Make tests minimal. Only test exactly the pattern being transformed. If your
+original motivating case is a larger pattern that your fold enables to
+optimize in some non-trivial way, you may add it as well -- however, the bulk
+of the test coverage should be minimal.
----------------
dtcxzyw wrote:

TODO: Drop unused poison-generating flags/UB-implying attributes.

https://github.com/llvm/llvm-project/pull/79007