[llvm] Reland "[SelectionDAG] Introducing a new ISD::POISON SDNode to represent the poison value in the IR." (PR #135056)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> FYI, for reference, I've bisected a very elusive and tricky crash down to this commit, when compiling with Clang on Windows/arm64. Unfortunately the crash isn't fully deterministic and seems dependent on what version of Clang I'm using to build Clang, so it's not clear cut what's the root cause. But posting here in case someone else also investigates and looks at bisects that point at this commit.

Sorry for the noise; this commit was unrelated to the issue. (This commit only uncovered the bug that had been present for a couple weeks before.)

https://github.com/llvm/llvm-project/pull/135056