test: check const-fold-gep with poison, undef, id

[artagnon]

For completeness, check poison, undef, and identity folding with GEP.

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 95c0c784ac9a91a8e12331ad9574ac6ad75318b1 7c9a5e380c8af99bd09c9258d178f89230dd73e3 llvm/test/Other/constant-fold-gep.ll

The following files introduce new uses of undef:

• llvm/test/Other/constant-fold-gep.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

[nikic]

Did you confirm that none of these cases are tested elsewhere?

I'd prefer not to extend this non-UTC test unless necessary.

[artagnon]

I looked at 1383cb6 ([ConstFold] Fix incorrect gep inbounds of undef fold), and it only seems to be tested indirectly in InstSimplify/ConstProp. As far as I can tell, there is no direct test, but I fully understand if you don't want to extend a non-UTC test.

[nikic]

InstSimplify/ConstProp is the usual way to test these code paths...