[InstSimplify] Simplify select i1 ConstExpr, i1 true, i1 false to ConstExpr

[StephenFan]

Summary

select i1 non-const, i1 true, i1 false has been optimized to
non-const. There is no reason that we can not optimize `select i1
ConstExpr, i1 true, i1 false` to ConstExpr.

[nikic]

llvm/test/Transforms/InstSimplify/select.ll

892

Outdated comment. We should also have a test where the constant expression is poison-generating.

[StephenFan]

Add poison generating constexpr test

[nikic]

LGTM

llvm/test/Transforms/InstSimplify/select.ll

892

Outdated comments are still there.

[khei4]

llvm/test/Transforms/InstSimplify/select-inseltpoison.ll

890

I think
i32 ptrtoint (ptr getelementptr inbounds ([3 x ptr], ptr @a, i64 0, i64 1) to i32) doesn't produce poison.
index seems valid so maybe
i32 ptrtoint (ptr getelementptr inbounds ([3 x ptr], ptr @a, i64 1, i64 0) to i32) ?

[nikic]

llvm/test/Transforms/InstSimplify/select-inseltpoison.ll

890

Yes, strictly speaking this is also safe to fold, but would require analysis we don't currently do.

To make it actually poison you'd need something like i32 ptrtoint (ptr getelementptr inbounds ([3 x ptr], ptr @a, i64 2) to i32).

[StephenFan]

Update test.