[ValueTracking] Handle returned attribute with mismatched type

The returned attribute can be used when it is possible to
"losslessly bitcast" between the argument and return type,
including between two vector types.

computeKnownBits() would crash in this case, isKnownNonZero()
would potentially produce a miscompile.

Fixes https://github.com/llvm/llvm-project/issues/74722.