[llvm] [SelectionDAG] Improve isGuaranteedNotToBeUndefOrPoison (PR #147019)

[Björn Pettersson via llvm-commits]

bjope wrote:

> Is the peekThroughInsertVectorElt code necessary for the test changes? I'm not opposed to adding handling for many opcodes without much test coverage but the INSERT_VECTOR_ELT code looks to be a very special case.

The motivation for peekThroughInsertVectorElt is test/CodeGen/X86/mmx-build-vector.ll (see https://github.com/llvm/llvm-project/pull/143103/files#diff-c897e7a69774c6782f9381e10373320d43d5a3bb0a90746a465a80fe67629e3cL485).
There is a sequence with insert_vector_elt to initialize a vector and we would suffer from the recursion depth limit if not peeking.

https://github.com/llvm/llvm-project/pull/147019