[InstCombine] Remove dead poison check. NFCI (#141264)
As far as I understand any binary op with poison as either operand will
constant fold to poison, so this check will never trigger.
`llvm::ConstantFoldBinaryInstruction` seems to confirm this?
I think this ended up getting left behind because originally
shufflevectors with undef indices produced undef elements, and we
couldn't pull the shuffle across some binops like `or undef, -1 --> -1`.
This code was added in 8c655150827b5d56772e628994db08441c554097 to
partially fix it and further extended in
f7499011ca29bebeda7c9d79d79b290cf0b8b46d, originally checking for undef
but changed to check for poison in cd54c47424456
But nowadays shufflevectors with undef indices are treated as poison
indices as of 575fdea70a86f68b0d303a9a3273fc47f810628a, and so produce
poison elements, so this is no longer an issue
