[llvm] [InstCombine][Docs] Update InstCombine contributor guide (PR #144228)

[Nikita Popov via llvm-commits]

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@@ -404,11 +404,32 @@ The use of TargetTransformInfo is only allowed for hooks for target-specific
 intrinsics, such as `TargetTransformInfo::instCombineIntrinsic()`. These are
 already inherently target-dependent anyway.
 
+If some canonicalization narrow/widen the integer width of expressions, please
+check `shouldChangeType()` first. Otherwise, we may evaluate the expression 
+in illegal/inefficient types. For vector types, follow the instructions below.
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nikic wrote:

```suggestion
in illegal/inefficient types.
```
It's in the next sentence, so...

https://github.com/llvm/llvm-project/pull/144228