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

[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 prior to your fix.
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goldsteinn wrote:

'prior to your fix' sounds a bit unclear imo. How about 'without your implementation'?

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