[SLPVectorizer] Hang with phi nodes

[nikic]

This input is reduced to run for approximately one second: https://gist.github.com/nikic/dd7a20a6604e7ab3f4c9ebce8377c2c9

This is the original input, which hangs ~forever (I stopped waiting after a few minutes): https://gist.github.com/nikic/f12002e855c733cece331e9d4d2841f3

[nikic]

Confirmed this fixes the original issue when backported to LLVM 21.

[nikic]

/cherry-pick 7730ebc

[llvmbot]

/pull-request #156297